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SubjectRe: unlink(nonexistent): EROFS or ENOENT?
Thank you for the answer.  I thought noone will reply... ;)

06.06.2011 07:39, Ted Ts'o wrote:
> On Sun, May 29, 2011 at 08:08:55PM +0400, Michael Tokarev wrote:
>> Hello.
>>
>> Just noticed that at least on ext4, unlinking a
>> non-existing file from a read-only filesystem
>> results in EROFS instead of ENOENT. I'd expect
>> it return ENOENT - it is more logical, at least
>> in my opinion.
>>
>> For one, (readonly) NFS mount returns ENOENT in
>> this case.
>
> Um, it doesn't for me. Testing on v3.0-rc1:
>
> # ls /test/foo; rm /test/foo
> ls: cannot access /test/foo: No such file or directory
> rm: cannot remove `/test/foo': No such file or directory

This is a hack in coreutils rm to work around this
kernel change. The comment at
http://git.savannah.gnu.org/cgit/coreutils.git/tree/src/remove.c#n450
says:

/* The unlinkat from kernels like linux-2.6.32 reports EROFS even for
nonexistent files. When the file is indeed missing, map that to ENOENT,
so that rm -f ignores it, as required. Even without -f, this is useful
because it makes rm print the more precise diagnostic. */
so that rm(1) calls stat(2) to see if the file actually
exist if unlinkat() returned EROFS, and turns this errno
into ENOENT.

That is, rm(1) output is not a good indicator. Use

strace rm -f /test/foo 2>&1 | grep unlink
to see the actual errno reported by the kernel.

Here's the POSIX description of unlink (and unlinkat) again:
http://pubs.opengroup.org/onlinepubs/009695399/functions/unlink.html

Thanks!

/mjt


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