|Summary:||dprintf fails to be async-signal safe|
|Product:||glibc||Reporter:||Rich Felker <bugdal>|
|Component:||stdio||Assignee:||Not yet assigned to anyone <unassigned>|
Description Rich Felker 2013-10-17 19:31:27 UTC
This is NOT a conformance bug, as POSIX does not require dprintf to be AS-safe or even mention it as an option. However, Roland McGrath has stated on libc-alpha that, as the original inventor of this interface, he intended it to be AS-safe and in general not to have unnecessary failure cases. At present, dprintf fails to be AS-safe for at least the following reasons: 1. It adds and removes the temporary FILE structure it creates to/from the global open streams list, requiring a lock. This operation is entirely unnecessary and nonsensical (and also documented in bug #12847). 2. The printf core uses malloc in various places (wide string conversion, i18n %n$ arguments, ...). Fixing these uses is less trivial, and may not matter to the most common usage cases for dprintf which are unlikely to hit the code paths that use malloc, but they are bugs in themselves anyway (unnecessary failure cases for the entire printf family) which should be fixed. There may also be other AS-safety issues I am unaware of.
Comment 1 email@example.com 2020-07-07 14:54:49 UTC
The master branch has been updated by Joseph Myers <firstname.lastname@example.org>: https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=6caddd34bd7ffb5ac4f36c8e036eee100c2cc535 commit 6caddd34bd7ffb5ac4f36c8e036eee100c2cc535 Author: Joseph Myers <email@example.com> Date: Tue Jul 7 14:54:12 2020 +0000 Remove most vfprintf width/precision-dependent allocations (bug 14231, bug 26211). The vfprintf implementation (used for all printf-family functions) contains complicated logic to allocate internal buffers of a size depending on the width and precision used for a format, using either malloc or alloca depending on that size, and with consequent checks for size overflow and allocation failure. As noted in bug 26211, the version of that logic used when '$' plus argument number formats are in use is missing the overflow checks, which can result in segfaults (quite possibly exploitable, I didn't try to work that out) when the width or precision is in the range 0x7fffffe0 through 0x7fffffff (maybe smaller values as well in the wprintf case on 32-bit systems, when the multiplication by sizeof (CHAR_T) can overflow). All that complicated logic in fact appears to be useless. As far as I can tell, there has been no need (outside the floating-point printf code, which does its own allocations) for allocations depending on width or precision since commit 3e95f6602b226e0de06aaff686dc47b282d7cc16 ("Remove limitation on size of precision for integers", Sun Sep 12 21:23:32 1999 +0000). Thus, this patch removes that logic completely, thereby fixing both problems with excessive allocations for large width and precision for non-floating-point formats, and the problem with missing overflow checks with such allocations. Note that this does have the consequence that width and precision up to INT_MAX are now allowed where previously INT_MAX / sizeof (CHAR_T) - EXTSIZ or more would have been rejected, so could potentially expose any other overflows where the value would previously have been rejected by those removed checks. I believe this completely fixes bugs 14231 and 26211. Excessive allocations are still possible in the floating-point case (bug 21127), as are other integer or buffer overflows (see bug 26201). This does not address the cases where a precision larger than INT_MAX (embedded in the format string) would be meaningful without printf's return value overflowing (when it's used with a string format, or %g without the '#' flag, so the actual output will be much smaller), as mentioned in bug 17829 comment 8; using size_t internally for precision to handle that case would be complicated by struct printf_info being a public ABI. Nor does it address the matter of an INT_MIN width being negated (bug 17829 comment 7; the same logic appears a second time in the file as well, in the form of multiplying by -1). There may be other sources of memory allocations with malloc in printf functions as well (bug 24988, bug 16060). From inspection, I think there are also integer overflows in two copies of "if ((width -= len) < 0)" logic (where width is int, len is size_t and a very long string could result in spurious padding being output on a 32-bit system before printf overflows the count of output characters). Tested for x86-64 and x86.