linux-kernel - Re: [REGRESSION] fuse: execve() fails with ETXTBSY due to async fuse

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Message-ID: <da17987a-b096-9ebb-f058-8eb91f15b560@fastmail.fm>
Date:   Mon, 14 Aug 2023 14:07:08 +0200
From:   Bernd Schubert <bernd.schubert@...tmail.fm>
To:     Miklos Szeredi <miklos@...redi.hu>,
        Jürg Billeter <j@...ron.ch>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Tycho Andersen <tycho@...ho.pizza>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        regressions@...ts.linux.dev
Subject: Re: [REGRESSION] fuse: execve() fails with ETXTBSY due to async
 fuse_flush



On 8/14/23 13:02, Miklos Szeredi wrote:
> On Mon, 14 Aug 2023 at 08:03, Jürg Billeter <j@...ron.ch> wrote:
>>
>> Since v6.3-rc1 commit 5a8bee63b1 ("fuse: in fuse_flush only wait if
>> someone wants the return code") `fput()` is called asynchronously if a
>> file is closed as part of a process exiting, i.e., if there was no
>> explicit `close()` before exit.
>>
>> If the file was open for writing, also `put_write_access()` is called
>> asynchronously as part of the async `fput()`.
>>
>> If that newly written file is an executable, attempting to `execve()`
>> the new file can fail with `ETXTBSY` if it's called after the writer
>> process exited but before the async `fput()` has run.
> 
> Thanks for the report.
> 
> At this point, I think it would be best to revert the original patch,
> since only v6.4 has it.
> 
> The original fix was already a workaround, and I don't see a clear
> path forward in this direction.  We need to see if there's better
> direction.
> 
> Ideas?

Is there a good reason to flush O_RDONLY?


fuse: Avoid flush for O_RDONLY

From: Bernd Schubert <bschubert@....com>

A file opened in read-only moded does not have data to be
flushed, so no need to send flush at all.

This also mitigates -EBUSY for executables, which is due to
async flush with commit 5a8bee63b1.

Fixes: 5a8bee63b1 (unless executable opened in rw)
Signed-off-by: Bernd Schubert <bschubert@....com>


index 89d97f6188e0..e058a6af6751 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -545,7 +545,8 @@ static int fuse_flush(struct file *file, fl_owner_t id)
         if (fuse_is_bad(inode))
                 return -EIO;
  
-       if (ff->open_flags & FOPEN_NOFLUSH && !fm->fc->writeback_cache)
+       if ((ff->open_flags & FOPEN_NOFLUSH && !fm->fc->writeback_cache) ||
+           ((file->f_flags & O_ACCMODE) == O_RDONLY))
                 return 0;
  
         fa = kzalloc(sizeof(*fa), GFP_KERNEL);