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* [glibc/release/2.34/master] Fix memmove call in vfprintf-internal.c:group_number
@ 2022-11-11 16:28 Florian Weimer
  0 siblings, 0 replies; only message in thread
From: Florian Weimer @ 2022-11-11 16:28 UTC (permalink / raw)
  To: glibc-cvs;h=6e8044e910600f71f4802dba2d105007af8428c3

commit 6e8044e910600f71f4802dba2d105007af8428c3
Author: Joseph Myers <>
Date:   Mon Nov 8 19:11:51 2021 +0000

    Fix memmove call in vfprintf-internal.c:group_number
    A recent GCC mainline change introduces errors of the form:
    vfprintf-internal.c: In function 'group_number':
    vfprintf-internal.c:2093:15: error: 'memmove' specified bound between 9223372036854775808 and 18446744073709551615 exceeds maximum object size 9223372036854775807 [-Werror=stringop-overflow=]
     2093 |               memmove (w, s, (front_ptr -s) * sizeof (CHAR_T));
          |               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    This is a genuine bug in the glibc code: s > front_ptr is always true
    at this point in the code, and the intent is clearly for the
    subtraction to be the other way round.  The other arguments to the
    memmove call here also appear to be wrong; w and s point just *after*
    the destination and source for copying the rest of the number, so the
    size needs to be subtracted to get appropriate pointers for the
    copying.  Adjust the memmove call to conform to the apparent intent of
    the code, so fixing the -Wstringop-overflow error.
    Now, if the original code were ever executed, a buffer overrun would
    result.  However, I believe this code (introduced in commit
    edc1686af0c0fc2eb535f1d38cdf63c1a5a03675, "vfprintf: Reuse work_buffer
    in group_number", so in glibc 2.26) is unreachable in prior glibc
    releases (so there is no need for a bug in Bugzilla, no need to
    consider any backports unless someone wants to build older glibc
    releases with GCC 12 and no possibility of this buffer overrun
    resulting in a security issue).
    work_buffer is 1000 bytes / 250 wide characters.  This case is only
    reachable if an initial part of the number, plus a grouped copy of the
    rest of the number, fail to fit in that space; that is, if the grouped
    number fails to fit in the space.  In the wide character case,
    grouping is always one wide character, so even with a locale (of which
    there aren't any in glibc) grouping every digit, a number would need
    to occupy at least 125 wide characters to overflow, and a 64-bit
    integer occupies at most 23 characters in octal including a leading 0.
    In the narrow character case, the multibyte encoding of the grouping
    separator would need to be at least 42 bytes to overflow, again
    supposing grouping every digit, but MB_LEN_MAX is 16.  So even if we
    admit the case of artificially constructed locales not shipped with
    glibc, given that such a locale would need to use one of the character
    sets supported by glibc, this code cannot be reached at present.  (And
    POSIX only actually specifies the ' flag for grouping for decimal
    output, though glibc acts on it for other bases as well.)
    With binary output (if you consider use of grouping there to be
    valid), you'd need a 15-byte multibyte character for overflow; I don't
    know if any supported character set has such a character (if, again,
    we admit constructed locales using grouping every digit and a grouping
    separator chosen to have a multibyte encoding as long as possible, as
    well as accepting use of grouping with binary), but given that we have
    this code at all (clearly it's not *correct*, or in accordance with
    the principle of avoiding arbitrary limits, to skip grouping on
    running out of internal space like that), I don't think it should need
    any further changes for binary printf support to go in.
    On the other hand, support for large sizes of _BitInt in printf (see
    the N2858 proposal) *would* require something to be done about such
    arbitrary limits (presumably using dynamic allocation in printf again,
    for sufficiently large _BitInt arguments only - currently only
    floating-point uses dynamic allocation, and, as previously discussed,
    that could actually be replaced by bounded allocation given smarter
    Tested with for aarch64-linux-gnu (GCC mainline).
    Also tested natively for x86_64.
    (cherry picked from commit db6c4935fae6005d46af413b32aa92f4f6059dce)

 stdio-common/vfprintf-internal.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/stdio-common/vfprintf-internal.c b/stdio-common/vfprintf-internal.c
index 3f3d1e148a..53d93b2f07 100644
--- a/stdio-common/vfprintf-internal.c
+++ b/stdio-common/vfprintf-internal.c
@@ -2154,7 +2154,8 @@ group_number (CHAR_T *front_ptr, CHAR_T *w, CHAR_T *rear_ptr,
 	      /* No further grouping to be done.  Copy the rest of the
 		 number.  */
-	      memmove (w, s, (front_ptr -s) * sizeof (CHAR_T));
+	      w -= s - front_ptr;
+	      memmove (w, front_ptr, (s - front_ptr) * sizeof (CHAR_T));
 	  else if (*grouping != '\0')

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