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

["patrickdepinguin at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

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

            Bug ID: 103173
           Summary: strncpy output may be truncated copying 32 bytes from
                    a string of length 1439 (bogus)
                    [-Werror=stringop-truncation]
           Product: gcc
           Version: 11.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: patrickdepinguin at gmail dot com
  Target Milestone: ---

gcc 11.2.0 gives a bogus warning of type 'stringop-truncation' on below reduced
test case.

.../arm-cortex_a53-linux-gnueabi-gcc -O2 -Wall -Werror -c /tmp/test.c -o
/tmp/foo.o                          
/tmp/test.c: In function 'func':
/tmp/test.c:22:5: error: 'strncpy' output may be truncated copying 32 bytes
from a string of length 1439 [-Werror=stringop-truncation]
   22 |     strncpy(dest, data[j].name, sizeof(dest));
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors


-----------------

#include <string.h>

struct data_struct {
    char name[32];
    int bar;
};

void func()
{
    extern struct data_struct data[40];
    extern char dest[32];

    int j=0;
    strncpy(dest, data[j].name, sizeof(dest));
};

-----------------


Here 'dest' is correctly interpreted as an array of size 32, but 'data[j].name'
is interpreted as having size 1439 which is incorrect, as it should also have
size 32 (name member of struct data_struct).

Following factors influence the reported size 1439:
* if data_struct is given more or less additional members, like 'bar'
* if the array length of 'data' (40) is changed
* if the length of the 'name' array in data_struct is changed

If 'j' is replaced by 0 directly, as in:
    strncpy(dest, data[0].name, sizeof(dest));
the error disappears.


This problem did not occur on our previous toolchain with gcc 7.4.0 for the
same architecture, nor for x86_64 with gcc 7.5.0.

Info from gcc -v:

Using built-in specs.
COLLECT_GCC=.../buildroot-toolchains/output/host/opt/ext-toolchain/bin/arm-cortex_a53-linux-gnueabi-gcc
COLLECT_LTO_WRAPPER=.../buildroot-toolchains/output/host/opt/ext-toolchain/arm/bin/../libexec/gcc/arm-cortex_a53-linux-gnueabi/11.2.0/lto-wrapper
Target: arm-cortex_a53-linux-gnueabi
Configured with:
.../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=.../crosstool-ng/targets/arm-cortex_a53-linux-gnueabi
--exec_prefix=.../crosstool-ng/targets/arm-cortex_a53-linux-gnueabi
--with-sysroot=.../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-libmudflap --disable-libgomp --disable-libssp
--disable-libquadmath --disable-libquadmath-support --disable-libsanitizer
--disable-libmpx
--with-gmp=.../crosstool-ng/.build/arm-cortex_a53-linux-gnueabi/buildtools
--with-mpfr=.../crosstool-ng/.build/arm-cortex_a53-linux-gnueabi/buildtools
--with-mpc=.../crosstool-ng/.build/arm-cortex_a53-linux-gnueabi/buildtools
--with-isl=.../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=.../crosstool-ng/targets/arm-cortex_a53-linux-gnueabi/arm-cortex_a53-linux-gnueabi/sysroot
--enable-long-long
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 11.2.0 (crosstool-NG 1.24.0.487_10ac846)