From mboxrd@z Thu Jan 1 00:00:00 1970 X-Msuck: nntp://news.gmane.org/gmane.linux.lib.musl.general/14258 Path: news.gmane.org!.POSTED.blaine.gmane.org!not-for-mail From: Rich Felker Newsgroups: gmane.linux.lib.musl.general Subject: Re: return-value/errno for utimensat(, NULL, NULL, 0) mismatch across musl and glibc: bug or a feature? Date: Tue, 25 Jun 2019 17:25:19 -0400 Message-ID: <20190625212519.GI1506@brightrain.aerifal.cx> References: <20190625213525.0407b535@sf> Reply-To: musl@lists.openwall.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Injection-Info: blaine.gmane.org; posting-host="blaine.gmane.org:195.159.176.226"; logging-data="201202"; mail-complaints-to="usenet@blaine.gmane.org" User-Agent: Mutt/1.5.21 (2010-09-15) Cc: musl@lists.openwall.com To: Sergei Trofimovich Original-X-From: musl-return-14274-gllmg-musl=m.gmane.org@lists.openwall.com Tue Jun 25 23:25:39 2019 Return-path: Envelope-to: gllmg-musl@m.gmane.org Original-Received: from mother.openwall.net ([195.42.179.200]) by blaine.gmane.org with smtp (Exim 4.89) (envelope-from ) id 1hfswc-000qEW-Pi for gllmg-musl@m.gmane.org; Tue, 25 Jun 2019 23:25:38 +0200 Original-Received: (qmail 28095 invoked by uid 550); 25 Jun 2019 21:25:36 -0000 Mailing-List: contact musl-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Original-Received: (qmail 28075 invoked from network); 25 Jun 2019 21:25:36 -0000 Content-Disposition: inline In-Reply-To: <20190625213525.0407b535@sf> Original-Sender: Rich Felker Xref: news.gmane.org gmane.linux.lib.musl.general:14258 Archived-At: On Tue, Jun 25, 2019 at 09:35:25PM +0100, Sergei Trofimovich wrote: > Hi musl@ folk! > > The original issue popped in https://bugs.gentoo.org/549108#c22. > There glibc's utimensat() wrapper handles one corner case differently > from musl's wrapper. > > Here is the minimal reproducer: > > $ cat a.c > #include > #include > #include > #include > > int main() { > int fd = open("f", O_WRONLY|O_CREAT, 0666); > return utimensat(fd, NULL, NULL, 0); > } > > On glibc (x86_64 linux-5.2-rc5): > > $ gcc a.c -o a && strace -etrace=open,openat,utimensat,exit_group ./a > openat(AT_FDCWD, "/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3 > openat(AT_FDCWD, "/lib64/libc.so.6", O_RDONLY|O_CLOEXEC) = 3 > openat(AT_FDCWD, "f", O_WRONLY|O_CREAT, 0666) = 3 > exit_group(-1) = ? > +++ exited with 255 +++ > > On musl (x86_64 linux-5.2-rc5): > $ gcc a.c -o a && strace -etrace=open,openat,utimensat,exit_group ./a > open("f", O_WRONLY|O_CREAT, 0666) = 3 > utimensat(3, NULL, NULL, 0) = 0 > exit_group(0) = ? > > The difference stems from this extra check in glibc: > https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/utimensat.c;h=04b549f360b88a7e7c1e5e617158caf73299736b;hb=HEAD#l32 > > int utimensat (int fd, const char *file, const struct timespec tsp[2], int flags) > { > if (file == NULL) > return INLINE_SYSCALL_ERROR_RETURN_VALUE (EINVAL); > /* Avoid implicit array coercion in syscall macros. */ > return INLINE_SYSCALL (utimensat, 4, fd, file, &tsp[0], flags); > } > > while musl just calls the syscall directly: > > https://git.musl-libc.org/cgit/musl/tree/src/stat/utimensat.c > > int utimensat(int fd, const char *path, const struct timespec times[2], int flags) > { > int r = __syscall(SYS_utimensat, fd, path, times, flags); > // ... > return __syscall_ret(r); > } > > Is this divergence expected? Or maybe it's accidental? Does it make > sense to handle non-directory fds in utimensat() according to POSIX? > > I wonder if we should drop the unstable test or some of libc implementations > actually deviates from the spec. I think the test is wrong. Passing a null pointer for a pathname argument where the interface requires a pointer to a string is undefined behavior unless the specification assigns special meaning to the null argument, and here it doesn't: http://pubs.opengroup.org/onlinepubs/9699919799/functions/utimensat.html The EINVAL error is specified for different purposes, so glibc is "wrong" on that too. Of course the "wrongness" isn't non-conforming, because anything can happen on UB, but if they want to catch it the error should probably be EFAULT for consistency. If there's a concern that the musl behavior is allowing silent incorrect behavior (operating on the fd argument rather than treating it as a directory for the relative "at" operation), perhaps we should either make it crash explicitly or simply do something like path?path:(void*)-1 to produce EFAULT and still show the wrong operation in strace. However I kinda don't like this since it makes implementing futimens in terms of utimensat more roundabout -- we'd have to introduce an extra internal symbol to get around the check. Rich