mmc-utils: fix potential overflow

Building with _FORTIFY_SOURCE=3 results in:
                 from mmc_cmds.c:20:
In function ‘read’,
    inlined from ‘do_rpmb_write_key’ at mmc_cmds.c:2233:8:
..unistd.h:38:10: error: ‘__read_alias’ writing 228 or more bytes into a region of size 32 overflows the destination [-Werror=stringop-overflow=]
   38 |   return __glibc_fortify (read, __nbytes, sizeof (char),
      |          ^~~~~~~~~~~~~~~
mmc_cmds.c: In function ‘do_rpmb_write_key’:
mmc_cmds.c:2087:19: note: destination object ‘key_mac’ of size 32
 2087 |         u_int8_t  key_mac[32];
      |                   ^~~~~~~
..unistd.h:26:16: note: in a call to function ‘__read_alias’ declared with attribute ‘access (write_only, 2, 3)’
   26 | extern ssize_t __REDIRECT (__read_alias, (int __fd, void *__buf,
      |                ^~~~~~~~~~

To work around this let's check if the return of read() is lower than the
nbyte requested instead of not equal.

Signed-off-by: Giulio Benetti <>
Reviewed-by: Avri Altman <>
Signed-off-by: Ulf Hansson <>
1 file changed