[Bug libc/31286] New: glibc 2.36: Aliasing violation in libio/iovdprintf.c before refactor to internal buffers

[Bug libc/31286] New: glibc 2.36: Aliasing violation in libio/iovdprintf.c before refactor to internal buffers

[carlos at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=31286

            Bug ID: 31286
           Summary: glibc 2.36: Aliasing violation in libio/iovdprintf.c
                    before refactor to internal buffers
           Product: glibc
           Version: 2.36
            Status: NEW
          Severity: normal
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: carlos at redhat dot com
                CC: drepper.fsp at gmail dot com
  Target Milestone: ---

In libio/iovdprintf.c in glibc 2.36 (before
8ece45e4f586abd212d1c02d74d38ef681a45600 and the conversion to internal
buffers):

 58 
 59   _IO_FINISH (&tmpfil.file);
 60 

The macro casts from the first member to the outer containing struct.

 92 /* Essentially ((TYPE *) THIS)->MEMBER, but avoiding the aliasing
 93    violation in case THIS has a different pointer type.  */
 94 #define _IO_CAST_FIELD_ACCESS(THIS, TYPE, MEMBER) \
 95   (*(_IO_MEMBER_TYPE (TYPE, MEMBER) *)(((char *) (THIS)) \
 96                                        + offsetof(TYPE, MEMBER)))

This works but some downstream pointer provenance checking tooling raises this
as an error.

It really should have been '_IO_FINISH (&tmpfil)'

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[Bug libc/31286] glibc 2.36: Aliasing violation in libio/iovdprintf.c before refactor to internal buffers

[sam at gentoo dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=31286

Sam James <sam at gentoo dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |sam at gentoo dot org

--- Comment #1 from Sam James <sam at gentoo dot org> ---
> This works but some downstream pointer provenance checking tooling raises
> this as an error.

(Out of interest, what tooling?)

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[Bug libc/31286] glibc 2.36: Aliasing violation in libio/iovdprintf.c before refactor to internal buffers

[carlos at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=31286

--- Comment #2 from Carlos O'Donell <carlos at redhat dot com> ---
(In reply to Sam James from comment #1)
> > This works but some downstream pointer provenance checking tooling raises
> > this as an error.
> 
> (Out of interest, what tooling?)

Coverity.

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[Bug libc/31286] glibc 2.36: Aliasing violation in libio/iovdprintf.c before refactor to internal buffers

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=31286

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |fweimer at redhat dot com

--- Comment #3 from Florian Weimer <fweimer at redhat dot com> ---
_IO_new_fdopen has a similar problem:

      off64_t new_pos = _IO_SYSSEEK (&new_f->fp.file, 0, _IO_seek_end);

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[Bug libc/31286] glibc 2.36: Aliasing violation in libio/iovdprintf.c before refactor to internal buffers

[sam at gentoo dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=31286

--- Comment #4 from Sam James <sam at gentoo dot org> ---
-Wstrict-aliasing=2 has many complaints as well with gcc version 14.0.1
20240214 (experimental) and -O3. I don't think all of them are real problems,
but at a glance, at least some of them are.

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