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posix_fallocate: Emulation fixes and documentation [BZ #15661]
Handle signed integer overflow correctly. Detect and reject O_APPEND. Document drawbacks of emulation. This does not completely address bug 15661, but improves the situation somewhat.
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@ -1,3 +1,12 @@
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2015-06-05 Florian Weimer <fweimer@redhat.com>
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[BZ #15661]
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* sysdeps/posix/posix_fallocate64.c (__posix_fallocate64_l64):
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Check for overflow properly. Check for O_APPEND. Ignore large
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file system block sizes. Add comments about problems.
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* sysdeps/posix/posix_fallocate.c (posix_fallocate): Likewise.
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* manual/filesys.texi (Storage Allocation): New node.
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2015-06-04 Adhemerval Zanella <adhemerval.zanella@linaro.org>
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* sysdeps/unix/sysdep.h [SYSCALL_CANCEL]: New macro: define
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@ -1723,6 +1723,7 @@ modify the attributes of a file.
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access a file.
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* File Times:: About the time attributes of a file.
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* File Size:: Manually changing the size of a file.
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* Storage Allocation:: Allocate backing storage for files.
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@end menu
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@node Attribute Meanings
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@ -3233,6 +3234,99 @@ is a requirement of @code{mmap}. The program has to keep track of the
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real size, and when it has finished a final @code{ftruncate} call should
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set the real size of the file.
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@node Storage Allocation
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@subsection Storage Allocation
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@cindex allocating file storage
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@cindex file allocation
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@cindex storage allocating
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@cindex file fragmentation
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@cindex fragmentation of files
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@cindex sparse files
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@cindex files, sparse
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Most file systems support allocating large files in a non-contiguous
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fashion: the file is split into @emph{fragments} which are allocated
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sequentially, but the fragments themselves can be scattered across the
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disk. File systems generally try to avoid such fragmentation because it
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decreases performance, but if a file gradually increases in size, there
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might be no other option than to fragment it. In addition, many file
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systems support @emph{sparse files} with @emph{holes}: regions of null
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bytes for which no backing storage has been allocated by the file
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system. When the holes are finally overwritten with data, fragmentation
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can occur as well.
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Explicit allocation of storage for yet-unwritten parts of the file can
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help the system to avoid fragmentation. Additionally, if storage
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pre-allocation fails, it is possible to report the out-of-disk error
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early, often without filling up the entire disk. However, due to
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deduplication, copy-on-write semantics, and file compression, such
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pre-allocation may not reliably prevent the out-of-disk-space error from
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occurring later. Checking for write errors is still required, and
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writes to memory-mapped regions created with @code{mmap} can still
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result in @code{SIGBUS}.
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@deftypefun int posix_fallocate (int @var{fd}, off_t @var{offset}, off_t @var{length})
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@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
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@c If the file system does not support allocation,
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@c @code{posix_fallocate} has a race with file extension (if
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@c @var{length} is zero) or with concurrent writes of non-NUL bytes (if
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@c @var{length} is positive).
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Allocate backing store for the region of @var{length} bytes starting at
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byte @var{offset} in the file for the descriptor @var{fd}. The file
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length is increased to @samp{@var{length} + @var{offset}} if necessary.
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@var{fd} must be a regular file opened for writing, or @code{EBADF} is
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returned. If there is insufficient disk space to fulfill the allocation
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request, @code{ENOSPC} is returned.
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@strong{Note:} If @code{fallocate} is not available (because the file
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system does not support it), @code{posix_fallocate} is emulated, which
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has the following drawbacks:
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@itemize @bullet
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@item
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It is very inefficient because all file system blocks in the requested
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range need to be examined (even if they have been allocated before) and
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potentially rewritten. In contrast, with proper @code{fallocate}
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support (see below), the file system can examine the internal file
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allocation data structures and eliminate holes directly, maybe even
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using unwritten extents (which are pre-allocated but uninitialized on
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disk).
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@item
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There is a race condition if another thread or process modifies the
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underlying file in the to-be-allocated area. Non-null bytes could be
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overwritten with null bytes.
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@item
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If @var{fd} has been opened with the @code{O_APPEND} flag, the function
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will fail with an @code{errno} value of @code{EBADF}.
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@item
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If @var{length} is zero, @code{ftruncate} is used to increase the file
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size as requested, without allocating file system blocks. There is a
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race condition which means that @code{ftruncate} can accidentally
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truncate the file if it has been extended concurrently.
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@end itemize
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On Linux, if an application does not benefit from emulation or if the
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emulation is harmful due to its inherent race conditions, the
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application can use the Linux-specific @code{fallocate} function, with a
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zero flag argument. For the @code{fallocate} function, @theglibc{} does
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not perform allocation emulation if the file system does not support
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allocation. Instead, an @code{EOPNOTSUPP} is returned to the caller.
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@end deftypefun
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@deftypefun int posix_fallocate64 (int @var{fd}, off64_t @var{length}, off64_t @var{offset})
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@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
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This function is a variant of @code{posix_fallocate64} which accepts
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64-bit file offsets on all platforms.
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@end deftypefun
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@node Making Special Files
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@section Making Special Files
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@cindex creating special files
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@ -18,26 +18,36 @@
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#include <errno.h>
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#include <fcntl.h>
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#include <unistd.h>
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#include <stdint.h>
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#include <sys/fcntl.h>
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#include <sys/stat.h>
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#include <sys/statfs.h>
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/* Reserve storage for the data of the file associated with FD. */
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/* Reserve storage for the data of the file associated with FD. This
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emulation is far from perfect, but the kernel cannot do not much
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better for network file systems, either. */
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int
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posix_fallocate (int fd, __off_t offset, __off_t len)
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{
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struct stat64 st;
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struct statfs f;
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/* `off_t' is a signed type. Therefore we can determine whether
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OFFSET + LEN is too large if it is a negative value. */
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if (offset < 0 || len < 0)
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return EINVAL;
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if (offset + len < 0)
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/* Perform overflow check. The outer cast relies on a GCC
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extension. */
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if ((__off_t) ((uint64_t) offset) + ((uint64_t) len) < 0)
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return EFBIG;
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/* First thing we have to make sure is that this is really a regular
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file. */
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/* pwrite below will not do the right thing in O_APPEND mode. */
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{
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int flags = __fcntl (fd, F_GETFL, 0);
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if (flags < 0 || (flags & O_APPEND) != 0)
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return EBADF;
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}
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/* We have to make sure that this is really a regular file. */
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if (__fxstat64 (_STAT_VER, fd, &st) != 0)
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return EBADF;
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if (S_ISFIFO (st.st_mode))
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@ -47,6 +57,8 @@ posix_fallocate (int fd, __off_t offset, __off_t len)
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if (len == 0)
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{
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/* This is racy, but there is no good way to satisfy a
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zero-length allocation request. */
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if (st.st_size < offset)
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{
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int ret = __ftruncate (fd, offset);
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@ -58,19 +70,36 @@ posix_fallocate (int fd, __off_t offset, __off_t len)
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return 0;
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}
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/* We have to know the block size of the filesystem to get at least some
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sort of performance. */
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if (__fstatfs (fd, &f) != 0)
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return errno;
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/* Minimize data transfer for network file systems, by issuing
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single-byte write requests spaced by the file system block size.
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(Most local file systems have fallocate support, so this fallback
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code is not used there.) */
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/* Try to play safe. */
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if (f.f_bsize == 0)
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f.f_bsize = 512;
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unsigned increment;
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{
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struct statfs64 f;
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/* Write something to every block. */
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for (offset += (len - 1) % f.f_bsize; len > 0; offset += f.f_bsize)
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if (__fstatfs64 (fd, &f) != 0)
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return errno;
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if (f.f_bsize == 0)
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increment = 512;
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else if (f.f_bsize < 4096)
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increment = f.f_bsize;
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else
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/* NFS does not propagate the block size of the underlying
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storage and may report a much larger value which would still
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leave holes after the loop below, so we cap the increment at
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4096. */
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increment = 4096;
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}
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/* Write a null byte to every block. This is racy; we currently
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lack a better option. Compare-and-swap against a file mapping
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might additional local races, but requires interposition of a
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signal handler to catch SIGBUS. */
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for (offset += (len - 1) % increment; len > 0; offset += increment)
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{
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len -= f.f_bsize;
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len -= increment;
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if (offset < st.st_size)
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{
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@ -18,26 +18,36 @@
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#include <errno.h>
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#include <fcntl.h>
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#include <unistd.h>
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#include <stdint.h>
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#include <sys/fcntl.h>
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#include <sys/stat.h>
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#include <sys/statfs.h>
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/* Reserve storage for the data of the file associated with FD. */
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/* Reserve storage for the data of the file associated with FD. This
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emulation is far from perfect, but the kernel cannot do not much
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better for network file systems, either. */
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int
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__posix_fallocate64_l64 (int fd, __off64_t offset, __off64_t len)
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{
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struct stat64 st;
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struct statfs64 f;
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/* `off64_t' is a signed type. Therefore we can determine whether
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OFFSET + LEN is too large if it is a negative value. */
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if (offset < 0 || len < 0)
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return EINVAL;
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if (offset + len < 0)
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/* Perform overflow check. The outer cast relies on a GCC
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extension. */
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if ((__off64_t) ((uint64_t) offset) + ((uint64_t) len) < 0)
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return EFBIG;
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/* First thing we have to make sure is that this is really a regular
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file. */
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/* pwrite64 below will not do the right thing in O_APPEND mode. */
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{
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int flags = __fcntl (fd, F_GETFL, 0);
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if (flags < 0 || (flags & O_APPEND) != 0)
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return EBADF;
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}
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/* We have to make sure that this is really a regular file. */
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if (__fxstat64 (_STAT_VER, fd, &st) != 0)
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return EBADF;
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if (S_ISFIFO (st.st_mode))
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@ -47,6 +57,8 @@ __posix_fallocate64_l64 (int fd, __off64_t offset, __off64_t len)
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if (len == 0)
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{
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/* This is racy, but there is no good way to satisfy a
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zero-length allocation request. */
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if (st.st_size < offset)
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{
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int ret = __ftruncate64 (fd, offset);
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@ -58,19 +70,36 @@ __posix_fallocate64_l64 (int fd, __off64_t offset, __off64_t len)
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return 0;
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}
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/* We have to know the block size of the filesystem to get at least some
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sort of performance. */
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if (__fstatfs64 (fd, &f) != 0)
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return errno;
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/* Minimize data transfer for network file systems, by issuing
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single-byte write requests spaced by the file system block size.
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(Most local file systems have fallocate support, so this fallback
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code is not used there.) */
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/* Try to play safe. */
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if (f.f_bsize == 0)
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f.f_bsize = 512;
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unsigned increment;
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{
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struct statfs64 f;
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/* Write something to every block. */
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for (offset += (len - 1) % f.f_bsize; len > 0; offset += f.f_bsize)
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if (__fstatfs64 (fd, &f) != 0)
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return errno;
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if (f.f_bsize == 0)
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increment = 512;
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else if (f.f_bsize < 4096)
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increment = f.f_bsize;
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else
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/* NFS clients do not propagate the block size of the underlying
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storage and may report a much larger value which would still
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leave holes after the loop below, so we cap the increment at
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4096. */
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increment = 4096;
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}
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/* Write a null byte to every block. This is racy; we currently
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lack a better option. Compare-and-swap against a file mapping
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might address local races, but requires interposition of a signal
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handler to catch SIGBUS. */
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for (offset += (len - 1) % increment; len > 0; offset += increment)
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{
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len -= f.f_bsize;
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len -= increment;
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if (offset < st.st_size)
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{
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