GNU bug report logs - #75405
gzip --synchronous doesn't work with musl

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Paul Eggert <eggert <at> cs.ucla.edu>
Cc: tracker <at> debbugs.gnu.org
Subject: bug#75405: closed (gzip --synchronous doesn't work with musl)
Date: Fri, 10 Jan 2025 18:20:02 +0000

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Your message dated Fri, 10 Jan 2025 10:19:32 -0800
with message-id <ba8cb748-cfca-4377-8695-976b25549d4d <at> cs.ucla.edu>
and subject line Re: bug#75405: gzip --synchronous doesn't work with musl
has caused the debbugs.gnu.org bug report #75405,
regarding gzip --synchronous doesn't work with musl
to be marked as done.

(If you believe you have received this mail in error, please contact
help-debbugs <at> gnu.org.)


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75405: https://debbugs.gnu.org/cgi/bugreport.cgi?bug=75405
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From: Lasse Collin <lasse.collin <at> tukaani.org>
To: bug-gzip <at> gnu.org
Subject: gzip --synchronous doesn't work with musl
Date: Mon, 6 Jan 2025 15:29:22 +0200

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On Linux/musl:

    $ echo foo > foo
    $ gzip --synchronous foo

    gzip: foo.gz: Bad file descriptor

From gzip.c:

      dfd = open (dfname, O_SEARCH | O_DIRECTORY);

...

        if ((synchronous
             && ((0 <= dfd && fdatasync (dfd) != 0 && errno != EINVAL)
                 || (fsync (ofd) != 0 && errno != EINVAL)))
            || close (ofd) != 0)
          write_error ();

In musl, O_SEARCH maps to Linux-specific O_PATH which doesn't allow
fsync or fdatasync. Gnulib's fcntl module's docs already warn about a
similar situation with fchmod.

As far as I understand this, O_SEARCH in POSIX is only meant for openat
and such APIs. Then it makes sense that O_SEARCH becomes O_PATH because
O_PATH can open a directory that lacks read permission. This way openat
can be used even for such directories.

Maybe there is no way to sync a directory that only has write and search
permissions. Luckily such a combination is rare.

A short fix is using O_RDONLY when --synchronous has been specified:

      dfd = open (dfname, (synchronous ? O_RDONLY : O_SEARCH)
                          | O_DIRECTORY);

I also tried opening a separate file descriptor for syncing (see the
attachment) but I think the above change is better. However, both have
a weakness that if the directory cannot be opened, --synchronous will
silently skip syncing the directory. It would be safer to show an error
if the directory cannot be opened when --synchronous has been specified.

-- 
Lasse Collin

[dfd_sync.patch (text/x-patch, attachment)]

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From: Paul Eggert <eggert <at> cs.ucla.edu>
To: Lasse Collin <lasse.collin <at> tukaani.org>
Cc: 75405-done <at> debbugs.gnu.org
Subject: Re: bug#75405: gzip --synchronous doesn't work with musl
Date: Fri, 10 Jan 2025 10:19:32 -0800

Thanks, I installed that test. Marking this gzip bug as done.

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