From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jeff Layton Date: Tue, 16 Aug 2016 19:41:50 -0400 Subject: [LTP] [PATCH] fcntl.2: F_OFD_XXX needs flock64 In-Reply-To: <2c79788f-c74a-49e8-fa81-0e9cf4e2d75f@gmail.com> References: <20160816115506.GA25201@rei.lan> <2c79788f-c74a-49e8-fa81-0e9cf4e2d75f@gmail.com> Message-ID: <1471390910.2680.20.camel@poochiereds.net> List-Id: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 8bit To: ltp@lists.linux.it On Wed, 2016-08-17 at 08:04 +1200, Michael Kerrisk (man-pages) wrote: > [Jeff, can you comment?] > > Hi Cyril, > > On 08/16/2016 11:55 PM, Cyril Hrubis wrote: > > > > If we pass struct flock to the F_OFD_XXX fcntl() it will fail with > > EINVAL with a 32bit binary. That is because glibc uses fcntl64() by > > default but the struct flock uses 32bit off_t for 32bit binaries (unless > > _FILE_OFFSET_BITS=64) and kernel always expect flock64 for F_OFD_XXX in > > fcntl64(). Hence kernel will read some garbage that is a few bytes after > > the 32bit flock structure in this case which will likely end up with the > > syscall returning EINVAL. > > Okay -- I confirm the problem you report. I'm just not sure that the > patch below is the best fix. So, to summarize: > > * On 64-bit, flock{} and flock64{} are the same structure. > * On 32-bit, flock{} and flock64{} are different. > * On 32-bit, F_OFD operations require flock64{}, but the traditional >   F_* lock operations do not. > * To use flock64{} with F_OFD operations, we can either explicitly use >   flock64{} or we can compile with -D_FILE_OFFSET_BITS=64 > > One solution would be your patch below, but it feels wrong: on 64-bit > flock{} suffices, and is consistent with the traditional F_* operations. > An alternative would be a note in the man page that says something along > the lines that on 32-bit, one must compile with -D_FILE_OFFSET_BITS=64 > when using the F_OFD operations. > > Your thoughts? > > Cheers, > > Michael > This sounds like a regular old bug, rather than a documentation issue.  The way the kernel works is that if you call fcntl(), then you need to pass in a struct flock. If you call fcntl64() then you need to pass in a struct flock64. Of course this is only on 32-bit arches. On 64-bit, it's there is no flock64 or fcntl64. Typically, glibc papers over all of this by deciding which syscall it's going to use based on -D_FILE_OFFSET_BITS. IIRC, it basically redefines the fields in struct flock to be like the one in struct flock64, so you shouldn't need to do anything special here. It sounds here like you got a mismatch, somehow and were calling fcntl64() with the smaller struct flock? Or was it vice versa? What would be ideal would be a small reproducer program, and instructions on how to build it. With that we should be able to nail down why this is happening. Also, what arch are you using here? > > > > Signed-off-by: Cyril Hrubis > > > > CC: Yuriy Kolerov > > --- > >  man2/fcntl.2 | 8 ++++---- > >  1 file changed, 4 insertions(+), 4 deletions(-) > > > > diff --git a/man2/fcntl.2 b/man2/fcntl.2 > > index f0c1acf..4606709 100644 > > --- a/man2/fcntl.2 > > +++ b/man2/fcntl.2 > > @@ -533,7 +533,7 @@ As with traditional advisory locks, the third argument to > >  .BR fcntl (), > >  .IR lock , > >  is a pointer to an > > -.IR flock > > +.IR flock64 > >  structure. > >  By contrast with traditional record locks, the > >  .I l_pid > > @@ -543,7 +543,7 @@ when using the commands described below. > >  The commands for working with open file description locks are analogous > >  to those used with traditional locks: > >  .TP > > -.BR F_OFD_SETLK " (\fIstruct flock *\fP)" > > +.BR F_OFD_SETLK " (\fIstruct flock64 *\fP)" > >  Acquire an open file description lock (when > >  .I l_type > >  is > > @@ -564,7 +564,7 @@ this call returns \-1 and sets > >  to > >  .BR EAGAIN . > >  .TP > > -.BR F_OFD_SETLKW " (\fIstruct flock *\fP)" > > +.BR F_OFD_SETLKW " (\fIstruct flock64 *\fP)" > >  As for > >  .BR F_OFD_SETLK , > >  but if a conflicting lock is held on the file, then wait for that lock to be > > @@ -578,7 +578,7 @@ set to > >  see > >  .BR signal (7)). > >  .TP > > -.BR F_OFD_GETLK " (\fIstruct flock *\fP)" > > +.BR F_OFD_GETLK " (\fIstruct flock64 *\fP)" > >  On input to this call, > >  .I lock > >  describes an open file description lock we would like to place on the file. > > > > -- Jeff Layton