Date: Thu, 16 Feb 2012 01:55:54 +1100 (EST) From: Bruce Evans <brde@optusnet.com.au> To: Nicolas Bourdaud <nicolas.bourdaud@gmail.com> Cc: freebsd-bugs@freebsd.org, freebsd-gnats-submit@freebsd.org, Bruce Evans <brde@optusnet.com.au> Subject: Re: kern/164793: 'write' system call violates POSIX standard Message-ID: <20120216005011.E2689@besplex.bde.org> In-Reply-To: <4F3BAF7B.2010305@gmail.com> References: <201202051142.q15Bgrh6041302@red.freebsd.org> <20120206050042.E2728@besplex.bde.org> <4F3BAF7B.2010305@gmail.com>
next in thread | previous in thread | raw e-mail | index | archive | help
On Wed, 15 Feb 2012, Nicolas Bourdaud wrote: > On 05/02/2012 19:54, Bruce Evans wrote: >> I think this is actually a bug in POSIX (XSI). Most programs aren't >> prepared to deal with short writes, and returning an error like >> truncate() is specified to is adequate. > > I disagree, I think that most programs that check that the write > succeeded also check that the write was complete. Actually it was Well, in BSD, programs that don't understand short writes start with the cp utility in 4.4BSD (it checks for short writes, but then mishandles them by treating them as errors). This wasn't fixed in FreeBSD until 1998. > because my programs were assuming the POSIX behavior that I notice the > bug. In addition, I think (this must be confirmed) that the bug don't > affect the version 8.2... So the programs are already facing the POSIX No, it was in 4.4BSD, and hasn't been changed in FreeBSD since 1994. 8.2 only differs in having the check in all file systems instead of in vfs. Perhaps some file systems got it right, but ffs didn't. > behavior. Moreover the programs that are cross platform (in particular > ported to Linux) are already facing this behavior. > > Whatever is decided, either freebsd should conform to the POSIX > standard, either the standard should be changed. It must conform, since it is too late to fix standards. I forgot about this when I looked at ffs's handling of i/o errors recently. There are many more bugs. ffs normally tries to back out of writes completely after an i/o error, by using ftruncate() to return to the original file size. Garbage written to the disk or memory is too hard to back out of, but ffs avoids security holes by zeroing it memory (in case it is memmap()ed) and by making it inaccessible by normal means on the disk (ftruncate() does this. When the error is ENOSPC due to a full disk, this gives the same behaviour as ffs has now for EFBIG for the file size being too big (due to the maximum size for the file system, or the rlimit). POSIX has looser wording for the ENOSPC error. It says that ENOSPC shall be returned if there "was" no space... This can be interpreted as requiring the same things as EFBIG -- that if there was any space to begin with, ENOSPC is not required to be returned; presumably the write() should succeed in writing as much as possible since there is no other reasonable error. But ffs's behaviour is "correct" here. The most broken case here is for an i/o error for a write in the middle of a file. Then it is not reasonable to try to back out. ffs doesn't do the ftruncate() in this case. But it still tries to back out. This results in write() returning -1/EIO. This is wrong if something has been successfully written. On second thoughts is it is the best possible behaviour. Everything in the region of the file covered by the write() may have been clobbered, either by writing the requested bytes, or by a hardware or software error writing garbage, or by the intentional zeroing for security. The only way to tell the application about this is to say that the whole write failed. The application should assume that the entire region has been clobbered, and take steps to check and limit the extent of the damage, perhaps by trying to rewrite it all in smaller pieces. There seem to be more bugs in [f]truncate(): - POSIX requires SIGXFSZ for attempts to exceed the file size rlimit in truncate() too, but FreeBSD doesn't even check the rlimit for truncate(). Checking the rlimit in vfs makes all this easier to fix. I think write() can be fixed in a couple of lines in vfs. All file systems call back to vfs to check, though I don't know of any requirement for other errors to have precedence, so vfs could check up front. zfs's write vnop actually calls back to vfs before doing anything else, so this error already has precedence over all fs-specific errors for zfs. All other file systems' write vnop do the check a fair way into the vnop in much the same place as ffs. No file systems check the limit for truncate(). The limit checking is commented out in xfs's write vnop. Bruce
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20120216005011.E2689>