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Make vmessage a bit safer

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* src/xdisp.c (vmessage): Avoid undefined behavior if
FRAME_MESSAGE_BUF_SIZE (f) is zero, or if doprnt generates output
containing only encoding errors.  Although it’s not clear whether
either is possible, it is better to be safe.  Also, clarify via a
new local message_bufsize.
This commit is contained in:
Paul Eggert 2025-01-27 17:13:02 -08:00
parent 8812f526cf
commit 8e7588a267
1 changed files with 5 additions and 4 deletions
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9
src/xdisp.c
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@ -12586,17 +12586,18 @@ vmessage (const char *m, va_list ap)
{
ptrdiff_t len;
ptrdiff_t maxsize = FRAME_MESSAGE_BUF_SIZE (f);
ptrdiff_t message_bufsize = maxsize + MAX_MULTIBYTE_LENGTH;
USE_SAFE_ALLOCA;
char *message_buf = SAFE_ALLOCA (maxsize + MAX_MULTIBYTE_LENGTH);
char *message_buf = SAFE_ALLOCA (message_bufsize);
len = doprnt (message_buf, maxsize + MAX_MULTIBYTE_LENGTH, m, 0, ap);
len = doprnt (message_buf, message_bufsize, m, 0, ap);
/* doprnt returns the buffer size minus one when it
truncated a multibyte sequence. Work around that by
truncating to the last valid multibyte head. */
if (len >= maxsize)
if (0 < maxsize && maxsize <= len)
{
len = maxsize - 1;
while (!CHAR_HEAD_P (message_buf[len]))
while (0 < len && !CHAR_HEAD_P (message_buf[len]))
len--;
message_buf[len] = 0;
}