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The GNU implementation of wcrtomb assumes that there are at least MB_CUR_MAX bytes available in the destination buffer passed to wcrtomb as the first argument. This is not compatible with the POSIX definition, which only requires enough space for the input wide character. This does not break much in practice because when users supply buffers smaller than MB_CUR_MAX (e.g. in ncurses), they compute and dynamically allocate the buffer, which results in enough spare space (thanks to usable_size in malloc and padding in alloca) that no actual buffer overflow occurs. However when the code is built with _FORTIFY_SOURCE, it runs into the hard check against MB_CUR_MAX in __wcrtomb_chk and hence fails. It wasn't evident until now since dynamic allocations would result in wcrtomb not being fortified but since _FORTIFY_SOURCE=3, that limitation is gone, resulting in such code failing. To fix this problem, introduce an internal buffer that is MB_LEN_MAX long and use that to perform the conversion and then copy the resultant bytes into the destination buffer. Also move the fortification check into the main implementation, which checks the result after conversion and aborts if the resultant byte count is greater than the destination buffer size. One complication is that applications that assume the MB_CUR_MAX limitation to be gone may not be able to run safely on older glibcs if they use static destination buffers smaller than MB_CUR_MAX; dynamic allocations will always have enough spare space that no actual overruns will occur. One alternative to fixing this is to bump symbol version to prevent them from running on older glibcs but that seems too strict a constraint. Instead, since these users will only have made this decision on reading the manual, I have put a note in the manual warning them about the pitfalls of having static buffers smaller than MB_CUR_MAX and running them on older glibc. Benchmarking: The wcrtomb microbenchmark shows significant increases in maximum execution time for all locales, ranging from 10x for ar_SA.UTF-8 to 1.5x-2x for nearly everything else. The mean execution time however saw practically no impact, with some results even being quicker, indicating that cache locality has a much bigger role in the overhead. Given that the additional copy uses a temporary buffer inside wcrtomb, it's likely that a hot path will end up putting that buffer (which is responsible for the additional overhead) in a similar place on stack, giving the necessary cache locality to negate the overhead. However in situations where wcrtomb ends up getting called at wildly different spots on the call stack (or is on different call stacks, e.g. with threads or different execution contexts) and is still a hotspot, the performance lag will be visible. Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org> |
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This directory contains the sources of the GNU C Library. See the file "version.h" for what release version you have. The GNU C Library is the standard system C library for all GNU systems, and is an important part of what makes up a GNU system. It provides the system API for all programs written in C and C-compatible languages such as C++ and Objective C; the runtime facilities of other programming languages use the C library to access the underlying operating system. In GNU/Linux systems, the C library works with the Linux kernel to implement the operating system behavior seen by user applications. In GNU/Hurd systems, it works with a microkernel and Hurd servers. The GNU C Library implements much of the POSIX.1 functionality in the GNU/Hurd system, using configurations i[4567]86-*-gnu. When working with Linux kernels, this version of the GNU C Library requires Linux kernel version 3.2 or later. Also note that the shared version of the libgcc_s library must be installed for the pthread library to work correctly. The GNU C Library supports these configurations for using Linux kernels: aarch64*-*-linux-gnu alpha*-*-linux-gnu arc*-*-linux-gnu arm-*-linux-gnueabi csky-*-linux-gnuabiv2 hppa-*-linux-gnu i[4567]86-*-linux-gnu x86_64-*-linux-gnu Can build either x86_64 or x32 ia64-*-linux-gnu m68k-*-linux-gnu microblaze*-*-linux-gnu mips-*-linux-gnu mips64-*-linux-gnu or1k-*-linux-gnu powerpc-*-linux-gnu Hardware or software floating point, BE only. powerpc64*-*-linux-gnu Big-endian and little-endian. s390-*-linux-gnu s390x-*-linux-gnu riscv32-*-linux-gnu riscv64-*-linux-gnu sh[34]-*-linux-gnu sparc*-*-linux-gnu sparc64*-*-linux-gnu If you are interested in doing a port, please contact the glibc maintainers; see https://www.gnu.org/software/libc/ for more information. See the file INSTALL to find out how to configure, build, and install the GNU C Library. You might also consider reading the WWW pages for the C library at https://www.gnu.org/software/libc/. The GNU C Library is (almost) completely documented by the Texinfo manual found in the `manual/' subdirectory. The manual is still being updated and contains some known errors and omissions; we regret that we do not have the resources to work on the manual as much as we would like. For corrections to the manual, please file a bug in the `manual' component, following the bug-reporting instructions below. Please be sure to check the manual in the current development sources to see if your problem has already been corrected. Please see https://www.gnu.org/software/libc/bugs.html for bug reporting information. We are now using the Bugzilla system to track all bug reports. This web page gives detailed information on how to report bugs properly. The GNU C Library is free software. See the file COPYING.LIB for copying conditions, and LICENSES for notices about a few contributions that require these additional notices to be distributed. License copyright years may be listed using range notation, e.g., 1996-2015, indicating that every year in the range, inclusive, is a copyrightable year that would otherwise be listed individually.