Re: Spurious warnings in crypto-des.c when building with gcc-16 -O3 - Mailing list pgsql-hackers

On Wed, 29 Apr 2026 at 23:17, Andres Freund <andres@anarazel.de> wrote:

Hi,

Recently I got a bit of a shock building postgres with gcc-16:

../../../../../home/andres/src/postgresql/contrib/pgcrypto/crypt-des.c: In function ‘px_crypt_des’:
../../../../../home/andres/src/postgresql/contrib/pgcrypto/crypt-des.c:675:22: warning: writing 8 bytes into a region of size 7 [-Wstringop-overflow=]
  675 |                 *q++ = *key << 1;
      |                 ~~~~~^~~~~~~~~~~
../../../../../home/andres/src/postgresql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [1, 2] into destination object ‘keybuf’ of size 8
  659 |                                 keybuf[2];
      |                                 ^~~~~~
../../../../../home/andres/src/postgresql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [2, 3] into destination object ‘keybuf’ of size 8
../../../../../home/andres/src/postgresql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [3, 4] into destination object ‘keybuf’ of size 8
../../../../../home/andres/src/postgresql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [4, 5] into destination object ‘keybuf’ of size 8
../../../../../home/andres/src/postgresql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [5, 6] into destination object ‘keybuf’ of size 8
../../../../../home/andres/src/postgresql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [6, 7] into destination object ‘keybuf’ of size 8
../../../../../home/andres/src/postgresql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [7, 8] into destination object ‘keybuf’ of size 8
../../../../../home/andres/src/postgresql/contrib/pgcrypto/crypt-des.c:675:22: warning: writing 1 byte into a region of size 0 [-Wstringop-overflow=]
  675 |                 *q++ = *key << 1;
      |                 ~~~~~^~~~~~~~~~~
../../../../../home/andres/src/postgresql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset 8 into destination object ‘keybuf’ of size 8
  659 |                                 keybuf[2];
      |                                 ^~~~~~
../../../../../home/andres/src/postgresql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [9, 10] into destination object ‘keybuf’ of size 8
../../../../../home/andres/src/postgresql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [10, 11] into destination object ‘keybuf’ of size 8
../../../../../home/andres/src/postgresql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [11, 12] into destination object ‘keybuf’ of size 8
../../../../../home/andres/src/postgresql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [12, 13] into destination object ‘keybuf’ of size 8
../../../../../home/andres/src/postgresql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [13, 14] into destination object ‘keybuf’ of size 8
../../../../../home/andres/src/postgresql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [14, 15] into destination object ‘keybuf’ of size 8


Luckily it turns out that that the warning is spurious, due to a bug in gcc
[1].

However, it took me quite a while to figure out what the hell the code was
doing:

char *
px_crypt_des(const char *key, const char *setting)
{
    uint32 keybuf[2];
...
    uint8      *q;
...

    /*
     * Copy the key, shifting each character up by one bit and padding with
     * zeros.
     */
    q = (uint8 *) keybuf;
    while (q - (uint8 *) keybuf - 8)
    {
        *q++ = *key << 1;
        if (*key != '\0')
            key++;
    }

Like, it's far from immediately obvious where the 8 is coming from (it's the
size of keybuf), whether there are precedence issues or what this is even
trying to achieve.

And it's still not clear to me why on earth it makes sense to write it that
complicated, when it seems something like

    for (int byteno = 0; byteno < sizeof(keybuf); byteno++)
    {
        *q++ = *key << 1;
        if (*key != '\0')
            key++;
    }

would do the same thing, except be trivially understandable for humans and
compilers.