From ae5062201d7e9d18fe88bff4bc71088374c394fb Mon Sep 17 00:00:00 2001 From: Siddhesh Poyarekar Date: Thu, 7 Nov 2024 11:16:04 -0500 Subject: [PATCH] ungetc: Guarantee single char pushback The C standard requires that ungetc guarantees at least one pushback, but the malloc call to allocate the pushback buffer could fail, thus violating that requirement. Fix this by adding a single byte pushback buffer in the FILE struct that the pushback can fall back to if malloc fails. The side-effect is that if the initial malloc fails and the 1-byte fallback buffer is used, future resizing (if it succeeds) will be 2-bytes, 4-bytes and so on, which is suboptimal but it's after a malloc failure, so maybe even desirable. A future optimization here could be to have the pushback code use the single byte buffer first and only fall back to malloc for subsequent calls. Signed-off-by: Siddhesh Poyarekar Reviewed-by: Maciej W. Rozycki --- libio/bits/types/struct_FILE.h | 5 +- libio/fileops.c | 7 +- libio/genops.c | 16 +++-- libio/libioP.h | 30 +++++--- libio/oldfileops.c | 5 +- libio/wfileops.c | 3 +- stdio-common/Makefile | 2 + stdio-common/tst-ungetc-nomem.c | 121 ++++++++++++++++++++++++++++++++ 8 files changed, 166 insertions(+), 23 deletions(-) create mode 100644 stdio-common/tst-ungetc-nomem.c diff --git a/libio/bits/types/struct_FILE.h b/libio/bits/types/struct_FILE.h index d8d26639d1..87197a328c 100644 --- a/libio/bits/types/struct_FILE.h +++ b/libio/bits/types/struct_FILE.h @@ -1,4 +1,5 @@ /* Copyright (C) 1991-2024 Free Software Foundation, Inc. + Copyright The GNU Toolchain Authors. This file is part of the GNU C Library. The GNU C Library is free software; you can redistribute it and/or @@ -70,7 +71,9 @@ struct _IO_FILE struct _IO_FILE *_chain; int _fileno; - int _flags2; + int _flags2:24; + /* Fallback buffer to use when malloc fails to allocate one. */ + char _short_backupbuf[1]; __off_t _old_offset; /* This used to be _offset but it's too small. */ /* 1+column number of pbase(); 0 is unknown. */ diff --git a/libio/fileops.c b/libio/fileops.c index 759d737ec7..d49e489f55 100644 --- a/libio/fileops.c +++ b/libio/fileops.c @@ -1,4 +1,5 @@ /* Copyright (C) 1993-2024 Free Software Foundation, Inc. + Copyright The GNU Toolchain Authors. This file is part of the GNU C Library. The GNU C Library is free software; you can redistribute it and/or @@ -480,7 +481,7 @@ _IO_new_file_underflow (FILE *fp) /* Maybe we already have a push back pointer. */ if (fp->_IO_save_base != NULL) { - free (fp->_IO_save_base); + _IO_free_backup_buf (fp, fp->_IO_save_base); fp->_flags &= ~_IO_IN_BACKUP; } _IO_doallocbuf (fp); @@ -932,7 +933,7 @@ _IO_new_file_seekoff (FILE *fp, off64_t offset, int dir, int mode) /* It could be that we already have a pushback buffer. */ if (fp->_IO_read_base != NULL) { - free (fp->_IO_read_base); + _IO_free_backup_buf (fp, fp->_IO_read_base); fp->_flags &= ~_IO_IN_BACKUP; } _IO_doallocbuf (fp); @@ -1282,7 +1283,7 @@ _IO_file_xsgetn (FILE *fp, void *data, size_t n) /* Maybe we already have a push back pointer. */ if (fp->_IO_save_base != NULL) { - free (fp->_IO_save_base); + _IO_free_backup_buf (fp, fp->_IO_save_base); fp->_flags &= ~_IO_IN_BACKUP; } _IO_doallocbuf (fp); diff --git a/libio/genops.c b/libio/genops.c index d7e35e67d5..02e159d6a8 100644 --- a/libio/genops.c +++ b/libio/genops.c @@ -1,4 +1,5 @@ /* Copyright (C) 1993-2024 Free Software Foundation, Inc. + Copyright The GNU Toolchain Authors. This file is part of the GNU C Library. The GNU C Library is free software; you can redistribute it and/or @@ -212,7 +213,7 @@ _IO_free_backup_area (FILE *fp) { if (_IO_in_backup (fp)) _IO_switch_to_main_get_area (fp); /* Just in case. */ - free (fp->_IO_save_base); + _IO_free_backup_buf (fp, fp->_IO_save_base); fp->_IO_save_base = NULL; fp->_IO_save_end = NULL; fp->_IO_backup_base = NULL; @@ -260,7 +261,7 @@ save_for_backup (FILE *fp, char *end_p) memcpy (new_buffer + avail, fp->_IO_read_base + least_mark, needed_size); - free (fp->_IO_save_base); + _IO_free_backup_buf (fp, fp->_IO_save_base); fp->_IO_save_base = new_buffer; fp->_IO_save_end = new_buffer + avail + needed_size; } @@ -636,7 +637,7 @@ _IO_default_finish (FILE *fp, int dummy) if (fp->_IO_save_base) { - free (fp->_IO_save_base); + _IO_free_backup_buf (fp, fp->_IO_save_base); fp->_IO_save_base = NULL; } @@ -998,11 +999,14 @@ _IO_default_pbackfail (FILE *fp, int c) else if (!_IO_have_backup (fp)) { /* No backup buffer: allocate one. */ - /* Use nshort buffer, if unused? (probably not) FIXME */ int backup_size = 128; char *bbuf = (char *) malloc (backup_size); if (bbuf == NULL) - return EOF; + { + /* Guarantee a 1-char pushback. */ + bbuf = fp->_short_backupbuf; + backup_size = 1; + } fp->_IO_save_base = bbuf; fp->_IO_save_end = fp->_IO_save_base + backup_size; fp->_IO_backup_base = fp->_IO_save_end; @@ -1022,7 +1026,7 @@ _IO_default_pbackfail (FILE *fp, int c) return EOF; memcpy (new_buf + (new_size - old_size), fp->_IO_read_base, old_size); - free (fp->_IO_read_base); + _IO_free_backup_buf (fp, fp->_IO_read_base); _IO_setg (fp, new_buf, new_buf + (new_size - old_size), new_buf + new_size); fp->_IO_backup_base = fp->_IO_read_ptr; diff --git a/libio/libioP.h b/libio/libioP.h index ad45579e13..714abbd549 100644 --- a/libio/libioP.h +++ b/libio/libioP.h @@ -1,4 +1,5 @@ /* Copyright (C) 1993-2024 Free Software Foundation, Inc. + Copyright The GNU Toolchain Authors. This file is part of the GNU C Library. The GNU C Library is free software; you can redistribute it and/or @@ -911,30 +912,30 @@ extern int _IO_vscanf (const char *, va_list) __THROW; # define FILEBUF_LITERAL(CHAIN, FLAGS, FD, WDP) \ { _IO_MAGIC+_IO_LINKED+_IO_IS_FILEBUF+FLAGS, \ NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, \ - NULL, NULL, (FILE *) CHAIN, FD, \ - 0, _IO_pos_BAD, 0, 0, { 0 }, &_IO_stdfile_##FD##_lock } + NULL, NULL, (FILE *) CHAIN, FD, 0, { 0 }, \ + _IO_pos_BAD, 0, 0, { 0 }, &_IO_stdfile_##FD##_lock } # else # define FILEBUF_LITERAL(CHAIN, FLAGS, FD, WDP) \ { _IO_MAGIC+_IO_LINKED+_IO_IS_FILEBUF+FLAGS, \ NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, \ - NULL, NULL, (FILE *) CHAIN, FD, \ - 0, _IO_pos_BAD, 0, 0, { 0 }, &_IO_stdfile_##FD##_lock, _IO_pos_BAD,\ - NULL, WDP, NULL } + NULL, NULL, (FILE *) CHAIN, FD, 0, { 0 }, \ + _IO_pos_BAD, 0, 0, { 0 }, &_IO_stdfile_##FD##_lock, \ + _IO_pos_BAD, NULL, WDP, NULL } # endif #else # ifdef _IO_USE_OLD_IO_FILE # define FILEBUF_LITERAL(CHAIN, FLAGS, FD, WDP) \ { _IO_MAGIC+_IO_LINKED+_IO_IS_FILEBUF+FLAGS, \ NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, \ - NULL, NULL, (FILE *) CHAIN, FD, \ - 0, _IO_pos_BAD } + NULL, NULL, (FILE *) CHAIN, FD, 0, { 0 }, \ + _IO_pos_BAD } # else # define FILEBUF_LITERAL(CHAIN, FLAGS, FD, WDP) \ { _IO_MAGIC+_IO_LINKED+_IO_IS_FILEBUF+FLAGS, \ NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, \ - NULL, NULL, (FILE *) CHAIN, FD, \ - 0, _IO_pos_BAD, 0, 0, { 0 }, NULL, _IO_pos_BAD, \ - NULL, WDP, NULL } + NULL, NULL, (FILE *) CHAIN, FD, 0, { 0 }, \ + _IO_pos_BAD, 0, 0, { 0 }, NULL, \ + _IO_pos_BAD, NULL, WDP, NULL } # endif #endif @@ -1040,6 +1041,15 @@ IO_validate_vtable (const struct _IO_jump_t *vtable) return vtable; } +/* In case of an allocation failure, we resort to using the fixed buffer + _SHORT_BACKUPBUF. Free PTR unless it points to that buffer. */ +static __always_inline void +_IO_free_backup_buf (FILE *fp, char *ptr) +{ + if (ptr != fp->_short_backupbuf) + free (ptr); +} + /* Character set conversion. */ enum __codecvt_result diff --git a/libio/oldfileops.c b/libio/oldfileops.c index 8f775c9094..03f4d76a57 100644 --- a/libio/oldfileops.c +++ b/libio/oldfileops.c @@ -1,4 +1,5 @@ /* Copyright (C) 1993-2024 Free Software Foundation, Inc. + Copyright The GNU Toolchain Authors. This file is part of the GNU C Library. The GNU C Library is free software; you can redistribute it and/or @@ -311,7 +312,7 @@ _IO_old_file_underflow (FILE *fp) /* Maybe we already have a push back pointer. */ if (fp->_IO_save_base != NULL) { - free (fp->_IO_save_base); + _IO_free_backup_buf (fp, fp->_IO_save_base); fp->_flags &= ~_IO_IN_BACKUP; } _IO_doallocbuf (fp); @@ -464,7 +465,7 @@ _IO_old_file_seekoff (FILE *fp, off64_t offset, int dir, int mode) /* It could be that we already have a pushback buffer. */ if (fp->_IO_read_base != NULL) { - free (fp->_IO_read_base); + _IO_free_backup_buf (fp, fp->_IO_read_base); fp->_flags &= ~_IO_IN_BACKUP; } _IO_doallocbuf (fp); diff --git a/libio/wfileops.c b/libio/wfileops.c index 16beab1f3a..a96bfa589b 100644 --- a/libio/wfileops.c +++ b/libio/wfileops.c @@ -1,4 +1,5 @@ /* Copyright (C) 1993-2024 Free Software Foundation, Inc. + Copyright The GNU Toolchain Authors. This file is part of the GNU C Library. The GNU C Library is free software; you can redistribute it and/or @@ -175,7 +176,7 @@ _IO_wfile_underflow (FILE *fp) /* Maybe we already have a push back pointer. */ if (fp->_IO_save_base != NULL) { - free (fp->_IO_save_base); + _IO_free_backup_buf (fp, fp->_IO_save_base); fp->_flags &= ~_IO_IN_BACKUP; } _IO_doallocbuf (fp); diff --git a/stdio-common/Makefile b/stdio-common/Makefile index e76e40e587..b1a04fd064 100644 --- a/stdio-common/Makefile +++ b/stdio-common/Makefile @@ -1,4 +1,5 @@ # Copyright (C) 1991-2024 Free Software Foundation, Inc. +# Copyright The GNU Toolchain Authors. # This file is part of the GNU C Library. # The GNU C Library is free software; you can redistribute it and/or @@ -303,6 +304,7 @@ tests := \ tst-tmpnam \ tst-ungetc \ tst-ungetc-leak \ + tst-ungetc-nomem \ tst-unlockedio \ tst-vfprintf-mbs-prec \ tst-vfprintf-user-type \ diff --git a/stdio-common/tst-ungetc-nomem.c b/stdio-common/tst-ungetc-nomem.c new file mode 100644 index 0000000000..0872de6050 --- /dev/null +++ b/stdio-common/tst-ungetc-nomem.c @@ -0,0 +1,121 @@ +/* Test ungetc behavior with malloc failures. + Copyright The GNU Toolchain Authors. + This file is part of the GNU C Library. + + The GNU C Library is free software; you can redistribute it and/or + modify it under the terms of the GNU Lesser General Public + License as published by the Free Software Foundation; either + version 2.1 of the License, or (at your option) any later version. + + The GNU C Library is distributed in the hope that it will be useful, + but WITHOUT ANY WARRANTY; without even the implied warranty of + MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU + Lesser General Public License for more details. + + You should have received a copy of the GNU Lesser General Public + License along with the GNU C Library; if not, see + . */ + +#include +#include +#include +#include +#include +#include +#include + +static volatile bool fail = false; + +/* Induce a malloc failure whenever FAIL is set; we use the __LIBC_MALLOC entry + point to avoid the other alternative, which is RTLD_NEXT. */ +void * +malloc (size_t sz) +{ + if (fail) + return NULL; + + static void *(*real_malloc) (size_t); + + if (real_malloc == NULL) + real_malloc = dlsym (RTLD_NEXT, "malloc"); + + return real_malloc (sz); +} + +static int +do_test (void) +{ + char *filename = NULL; + struct stat props = {}; + size_t bufsz = 0; + + create_temp_file ("tst-ungetc-nomem.", &filename); + if (stat (filename, &props) != 0) + FAIL_EXIT1 ("Could not get file status: %m\n"); + + FILE *fp = fopen (filename, "w"); + + /* The libio buffer sizes are the same as block size. This is to ensure that + the test runs at the read underflow boundary as well. */ + bufsz = props.st_blksize + 2; + + char *buf = xmalloc (bufsz); + memset (buf, 'a', bufsz); + + if (fwrite (buf, sizeof (char), bufsz, fp) != bufsz) + FAIL_EXIT1 ("fwrite failed: %m\n"); + xfclose (fp); + + /* Begin test. */ + fp = xfopen (filename, "r"); + + while (!feof (fp)) + { + /* Reset the pushback buffer state. */ + fseek (fp, 0, SEEK_CUR); + + fail = true; + /* 1: First ungetc should always succeed, as the standard requires. */ + TEST_COMPARE (ungetc ('b', fp), 'b'); + + /* 2: This will result in resizing, which should fail. */ + TEST_COMPARE (ungetc ('c', fp), EOF); + + /* 3: Now allow the resizing, which should immediately fill up the buffer + too, since this allocates only double the current buffer, i.e. + 2-bytes. */ + fail = false; + TEST_COMPARE (ungetc ('d', fp), 'd'); + + /* 4: And fail again because this again forces an alloc, which fails. */ + fail = true; + TEST_COMPARE (ungetc ('e', fp), EOF); + + /* 5: Enable allocations again so that we now get a 4-byte buffer. Now + both calls should work. */ + fail = false; + TEST_COMPARE (ungetc ('f', fp), 'f'); + fail = true; + TEST_COMPARE (ungetc ('g', fp), 'g'); + + /* Drain out the x's. */ + TEST_COMPARE (fgetc (fp), 'g'); + TEST_COMPARE (fgetc (fp), 'f'); + TEST_COMPARE (fgetc (fp), 'd'); + + /* Finally, drain out the first char we had pushed back, followed by one + more char from the stream, if present. */ + TEST_COMPARE (fgetc (fp), 'b'); + char c = fgetc (fp); + if (!feof (fp)) + TEST_COMPARE (c, 'a'); + } + + /* Final sanity check before we're done. */ + TEST_COMPARE (ferror (fp), 0); + xfclose (fp); + + return 0; +} + +#include