Re: Cleaning up pgcrypto/crypt-des.c to avoid compiler warnings - Mailing list pgsql-hackers

Hi,

On Fri, 25 Sept 2026 at 23:28, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> For some time now, buildfarm member serinus has been complaining
> about pgcrypto/crypt-des.c:
>
> [1601/2408] /usr/bin/ccache /usr/lib/gcc-snapshot/bin/gcc -Icontrib/pgcrypto/pgcrypto.so.p -Isrc/include -I../pgsql/src/include -I/usr/include/libxml2 -fdiagnostics-color=never -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -O2 -g -fno-strict-aliasing -fwrapv -fexcess-precision=standard -fpch-deps -D_GNU_SOURCE -Wpointer-arith -Werror=vla -Wmissing-format-attribute -Wcast-function-type -Wshadow=compatible-local -Wformat-security -Wmissing-prototypes -Wold-style-declaration -Wold-style-definition -Wstrict-prototypes -Wimplicit-fallthrough=5 -Wdeclaration-after-statement -Wmissing-variable-declarations -Wno-format-truncation -Wno-stringop-truncation -O1 -ggdb -g3 -fno-omit-frame-pointer -Wall -Wextra -Wno-unused-parameter -Wno-sign-compare -Wno-missing-field-initializers -O3 -Wno-unterminated-string-initialization -Wno-clobbered -fPIC -isystem /usr/include/mit-krb5 -isystem /usr/include/mit-krb5 -pthread -fvisibility=hidden -MD -MQ contrib/pgcrypto/pgcrypto.so.p/crypt-des.c.o -MF contrib/pgcrypto/pgcrypto.so.p/crypt-des.c.o.d -o contrib/pgcrypto/pgcrypto.so.p/crypt-des.c.o -c ../pgsql/contrib/pgcrypto/crypt-des.c
> ../pgsql/contrib/pgcrypto/crypt-des.c: In function 'px_crypt_des':
> ../pgsql/contrib/pgcrypto/crypt-des.c:675:22: warning: writing 8 bytes into a region of size 7 [-Wstringop-overflow=]
>   675 |                 *q++ = *key << 1;
>       |                 ~~~~~^~~~~~~~~~~
> ../pgsql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [1, 2] into destination object 'keybuf' of size 8
>   659 |                                 keybuf[2];
>       |                                 ^~~~~~
> ../pgsql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [2, 3] into destination object 'keybuf' of size 8
> ../pgsql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [3, 4] into destination object 'keybuf' of size 8
> ../pgsql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [4, 5] into destination object 'keybuf' of size 8
> ../pgsql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [5, 6] into destination object 'keybuf' of size 8
> ../pgsql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [6, 7] into destination object 'keybuf' of size 8
> ../pgsql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [7, 8] into destination object 'keybuf' of size 8
> ../pgsql/contrib/pgcrypto/crypt-des.c:675:22: warning: writing 1 byte into a region of size 0 [-Wstringop-overflow=]
>   675 |                 *q++ = *key << 1;
>       |                 ~~~~~^~~~~~~~~~~
> ../pgsql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset 8 into destination object 'keybuf' of size 8
>   659 |                                 keybuf[2];
>       |                                 ^~~~~~
> ../pgsql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [9, 10] into destination object 'keybuf' of size 8
> ../pgsql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [10, 11] into destination object 'keybuf' of size 8
> ../pgsql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [11, 12] into destination object 'keybuf' of size 8
> ../pgsql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [12, 13] into destination object 'keybuf' of size 8
> ../pgsql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [13, 14] into destination object 'keybuf' of size 8
> ../pgsql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [14, 15] into destination object 'keybuf' of size 8
>
> It's possible to see by inspection that this loop doesn't actually
> overrun the buffer.  But the termination condition is oddly written:
>
>         q = (uint8 *) keybuf;
>         while (q - (uint8 *) keybuf - 8)
>
> I think this odd coding, perhaps combined with all the cowboy casting
> that's going on here, is what's confusing gcc into giving a warning.
> I propose the attached patch to make this code less ugly and (with
> luck) suppress the warning.

Yeah I agree that it's pretty ugly.

We had an open patch for that obscure loop earlier[1].
(I'll close that commitfest item)

I reviewed your patch and it looks good and clearer to me.

Thanks for the patch.

Regards,
Ayush

[1] https://www.postgresql.org/message-id/CAJTYsWWkNO0fKZit%2B_hRUUgrP-Gyu6pGDT5w0w%2B4dYOM7HMF1Q%40mail.gmail.com