[Bug stdio/31183] New: Wide stream buffer size reduced MB_LEN_MAX bytes after bug 17522 fix

[Bug stdio/31183] New: Wide stream buffer size reduced MB_LEN_MAX bytes after bug 17522 fix

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=31183

            Bug ID: 31183
           Summary: Wide stream buffer size reduced MB_LEN_MAX bytes after
                    bug 17522 fix
           Product: glibc
           Version: 2.21
            Status: NEW
          Severity: normal
          Priority: P2
         Component: stdio
          Assignee: fweimer at redhat dot com
          Reporter: fweimer at redhat dot com
  Target Milestone: ---

After the fix from bug 17522, we do not use the remaining buffer space if the
already-written buffer space is smaller than MB_LEN_MAX (16 bytes).  As we
start out with no written bytes (zero), we never use the full buffer, greatly
reducing performance.

The fix probably explains it better:

-         if (fp->_IO_write_ptr - fp->_IO_write_base < sizeof (mb_buf))
+         if (fp->_IO_buf_end - fp->_IO_write_ptr < sizeof (mb_buf))
            {
              /* Make sure we have room for at least one multibyte
                 character.  */
              write_ptr = write_base = mb_buf;
              buf_end = mb_buf + sizeof (mb_buf);
            }

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[Bug stdio/31183] Wide stream buffer size reduced MB_LEN_MAX bytes after bug 17522 fix

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=31183

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
              Flags|                            |security-
                 CC|                            |fweimer at redhat dot com

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[Bug stdio/31183] Wide stream buffer size reduced MB_LEN_MAX bytes after bug 17522 fix

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=31183

--- Comment #1 from Florian Weimer <fweimer at redhat dot com> ---
Patch posted:

[PATCH] libio: Check remaining buffer size in _IO_wdo_write (bug 31183)
<https://inbox.sourceware.org/libc-alpha/87bkakx3b0.fsf@oldenburg.str.redhat.com/>

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[Bug stdio/31183] Wide stream buffer size reduced MB_LEN_MAX bytes after bug 17522 fix

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=31183

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
             Status|ASSIGNED                    |RESOLVED
   Target Milestone|---                         |2.39

--- Comment #2 from Florian Weimer <fweimer at redhat dot com> ---
Fixed for 2.39 via:

commit ecc7c3deb9f347649c2078fcc0f94d4cedf92d60
Author: Florian Weimer <fweimer@redhat.com>
Date:   Tue Jan 2 14:36:17 2024 +0100

    libio: Check remaining buffer size in _IO_wdo_write (bug 31183)

    The multibyte character needs to fit into the remaining buffer space,
    not the already-written buffer space.  Without the fix, we were never
    moving the write pointer from the start of the buffer, always using
    the single-character fallback buffer.

    Fixes commit 04b76b5aa8b2d1d19066e42dd1 ("Don't error out writing
    a multibyte character to an unbuffered stream (bug 17522)").

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