[PATCH v2] mmc-utils: fix potential overflow

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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:
/home/giuliobenetti/br_reproduce/a53922c5db3e605a5e81e53c034f45017ebb7db7/output/host/mipsel-buildroot-linux-gnu/sysroot/usr/include/bits/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];
      |                   ^~~~~~~
/home/giuliobenetti/br_reproduce/a53922c5db3e605a5e81e53c034f45017ebb7db7/output/host/mipsel-buildroot-linux-gnu/sysroot/usr/include/bits/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 <giulio.benetti@xxxxxxxxxxxxxxxxxxxxxx>
---
V1->V2:
* corrected commit log
---
 mmc_cmds.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mmc_cmds.c b/mmc_cmds.c
index 10d063d..ae7b876 100644
--- a/mmc_cmds.c
+++ b/mmc_cmds.c
@@ -2065,7 +2065,7 @@ int do_sanitize(int nargs, char **argv)
 			}										\
 			else if (r > 0)							\
 				ret += r;							\
-		} while (r != 0 && (size_t)ret != nbyte);	\
+		} while (r != 0 && (size_t)ret < nbyte);	\
 													\
 		ret;										\
 	})
-- 
2.34.1




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