From 9bcd12d223a8990254b65e2dada54faa5d2742f3 Mon Sep 17 00:00:00 2001 From: Siddhesh Poyarekar Date: Fri, 13 May 2022 19:10:15 +0530 Subject: wcrtomb: Make behavior POSIX compliant 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 --- debug/tst-fortify.c | 7 ++++++- debug/wcrtomb_chk.c | 8 ++------ 2 files changed, 8 insertions(+), 7 deletions(-) (limited to 'debug') diff --git a/debug/tst-fortify.c b/debug/tst-fortify.c index 03c9867714..8e94643bf2 100644 --- a/debug/tst-fortify.c +++ b/debug/tst-fortify.c @@ -1478,10 +1478,15 @@ do_test (void) character which has a multibyte representation which does not fit. */ CHK_FAIL_START - char smallbuf[2]; + char smallbuf[1]; if (wcrtomb (smallbuf, L'\x100', &s) != 2) FAIL (); CHK_FAIL_END + + /* Same input with a large enough buffer and we're good. */ + char bigenoughbuf[2]; + if (wcrtomb (bigenoughbuf, L'\x100', &s) != 2) + FAIL (); #endif wchar_t wenough[10]; diff --git a/debug/wcrtomb_chk.c b/debug/wcrtomb_chk.c index 8b6d026560..28c3ea0d2d 100644 --- a/debug/wcrtomb_chk.c +++ b/debug/wcrtomb_chk.c @@ -1,4 +1,5 @@ /* Copyright (C) 2005-2022 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 @@ -25,10 +26,5 @@ size_t __wcrtomb_chk (char *s, wchar_t wchar, mbstate_t *ps, size_t buflen) { - /* We do not have to implement the full wctomb semantics since we - know that S cannot be NULL when we come here. */ - if (buflen < MB_CUR_MAX) - __chk_fail (); - - return __wcrtomb (s, wchar, ps); + return __wcrtomb_internal (s, wchar, ps, buflen); } -- cgit 1.4.1