[Bug tree-optimization/103173] strncpy output may be truncated copying 32 bytes from a string of length 1439 (bogus) [-Werror=stringop-truncation]

[patrickdepinguin at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103173

--- Comment #3 from Thomas De Schampheleire <patrickdepinguin at gmail dot com> 
---
While the original test program failed on gcc 11.2.0 but not on gcc 9.4.0, I
now encounter a very similar case that does fail on gcc 9.4.0:

------------------------------
#include <stdio.h>

#define MAX_NR_USERS 10

struct user_data {
  char user[32];
  char password[32];
  char application[32];
};

struct user_data users[MAX_NR_USERS];

void login_process()
{
    char tmp_user[33];

    for (int i = 0; i < MAX_NR_USERS; i++)
    {
        snprintf(tmp_user, sizeof(tmp_user), "%s", &(users[i].user[0]));
    }
}
------------------------------


arm-cortex_a53-linux-gnueabi-gcc /tmp/gcc-9-test.c -c -Wall -O2                 
/tmp/gcc-9-test.c: In function 'login_process':
/tmp/gcc-9-test.c:19:47: warning: '%s' directive output may be truncated
writing up to 959 bytes into a region of size 33 [-Wformat-truncation=]
   19 |         snprintf(tmp_user, sizeof(tmp_user), "%s",
&(users[i].user[0]));
      |                                               ^~
/tmp/gcc-9-test.c:19:9: note: 'snprintf' output between 1 and 960 bytes into a
destination of size 33
   19 |         snprintf(tmp_user, sizeof(tmp_user), "%s",
&(users[i].user[0]));
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


Here, the claimed 960 bytes are the total size of 'users' (3 * 32 * 10), while
the copied 'user' field is only 33 byte and there should be no problem.
The error is now -Wformat-truncation instead of -Wstringop-truncation but
otherwise this looks to be the same underlying problem.

This compiler is:

Using built-in specs.
COLLECT_GCC=.../buildroot-toolchains-bis/output/host/opt/ext-toolchain/bin/arm-cortex_a53-linux-gnueabi-gcc
COLLECT_LTO_WRAPPER=.../buildroot-toolchains-bis/output/host/opt/ext-toolchain/arm/bin/../libexec/gcc/arm-cortex_a53-linux-gnueabi/9.4.0/lto-wrapper
Target: arm-cortex_a53-linux-gnueabi
Configured with:
.../ctng/crosstool-ng/.build/arm-cortex_a53-linux-gnueabi/src/gcc/configure
--build=x86_64-build_pc-linux-gnu --host=x86_64-build_pc-linux-gnu
--target=arm-cortex_a53-linux-gnueabi
--prefix=.../ctng/crosstool-ng/targets/arm-cortex_a53-linux-gnueabi
--exec_prefix=.../ctng/crosstool-ng/targets/arm-cortex_a53-linux-gnueabi
--with-sysroot=.../ctng/crosstool-ng/targets/arm-cortex_a53-linux-gnueabi/arm-cortex_a53-linux-gnueabi/sysroot
--enable-languages=c,c++,fortran --with-cpu=cortex-a53 --with-fpu=neon-fp-armv8
--with-float=hard --with-pkgversion='crosstool-NG 1.24.0.487_10ac846'
--enable-__cxa_atexit --disable-tm-clone-registry --disable-libmudflap
--disable-libgomp --disable-libssp --disable-libquadmath
--disable-libquadmath-support --disable-libsanitizer --disable-libmpx
--with-gmp=.../ctng/crosstool-ng/.build/arm-cortex_a53-linux-gnueabi/buildtools
--with-mpfr=.../ctng/crosstool-ng/.build/arm-cortex_a53-linux-gnueabi/buildtools
--with-mpc=.../ctng/crosstool-ng/.build/arm-cortex_a53-linux-gnueabi/buildtools
--with-isl=.../ctng/crosstool-ng/.build/arm-cortex_a53-linux-gnueabi/buildtools
--disable-lto --without-zstd --enable-threads=posix --enable-target-optspace
--disable-plugin --disable-nls --disable-multilib
--with-local-prefix=.../ctng/crosstool-ng/targets/arm-cortex_a53-linux-gnueabi/arm-cortex_a53-linux-gnueabi/sysroot
--enable-long-long
Thread model: posix
gcc version 9.4.0 (crosstool-NG 1.24.0.487_10ac846)