gcc: __fread_chk_warn warning - c

Question

I have a compilation problem with gcc.
Assume the following program:
#include <stdio.h>
int test(const char *fname) {
FILE *fh = fopen(fname, "rb");
int tmp;
if (fread(&tmp, sizeof(tmp), 1, fh) < 1) {
tmp = 0;
}
fclose(fh);
return tmp;
}
int main(void) {
printf("%d\n", test("test.txt"));
return 0;
}
And the file test.txt:
11111111111111111111111111111111...
Of course, the program is very stupid, but it works:
user#ubuntu:~/tmp/optxx$ gcc -O3 -Wall -Wextra test.c
user#ubuntu:~/tmp/optxx$ ./a.out
825307441
Let's modify it a bit (only add an attribute to the test-function):
#include <stdio.h>
int __attribute__((optimize("O0"))) test(const char *fname) {
FILE *fh = fopen(fname, "rb");
int tmp;
if (fread(&tmp, sizeof(tmp), 1, fh) < 1) {
tmp = 0;
}
fclose(fh);
return tmp;
}
int main(void) {
printf("%d\n", test("test.txt"));
return 0;
}
The function test should no longer be optimized. But now compilation fails:
user#ubuntu:~/tmp/optxx$ gcc -Wall -Wextra -O3 test.c
In file included from /usr/include/stdio.h:936:0,
from test.c:1:
In function ‘fread’,
inlined from ‘test’ at test.c:6:9:
/usr/include/x86_64-linux-gnu/bits/stdio2.h:293:9: warning: call to ‘__fread_chk_warn’ declared with attribute warning: fread called with bigger size * nmemb than length of destination buffer
return __fread_chk_warn (__ptr, __bos0 (__ptr), __size, __n, __stream);
^
I get a warning and normally i compile with -Werror, so i don't like warnings.
The used gcc version:
user#ubuntu:~/tmp/optxx$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/5/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 5.3.1-6ubuntu2' --with-bugurl=file:///usr/share/doc/gcc-5/README.Bugs --enable-languages=c,ada,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-5 --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-libmpx --enable-plugin --with-system-zlib --disable-browser-plugin --enable-java-awt=gtk --enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-5-amd64/jre --enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-5-amd64 --with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-5-amd64 --with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar --enable-objc-gc --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 5.3.1 20160119 (Ubuntu 5.3.1-6ubuntu2)
Unfortunately i am not able to fix the warning:(
Maybe someone has an idea why this happens?
-edit-
A (dirty) hack would also be okay to remove this warning:)
-edit-
Probably something with my linux is wrong, because it seems to work for everyone else. Anayway a dirty hack / fix would be to add this code before the fread call:
static size_t fread_wrapper(void *ptr, size_t size, size_t count, FILE *stream) {
return fread(ptr, size, count, stream);
}
#define fread(ptr, size, count, stream) fread_wrapper((ptr), (size), (count), (stream))

Answer 1

Ubuntu enable the _FORTIFY_SOURCE feature by default for optimization levels -O1 or higher. This option makes the system headers pre-process to use different functions for fread among others. Those functions perform some basic safety checks of arguments. The option gets enabled globally when the header is included so it makes the assumption that all the code will be compiled with -O1 or higher. I assume that it is disabled on lower optimization levels because gcc doesn't have enough information without some optimizations and that causes those magic fortify macros to generate false positives (like you got).
If you compile the whole file with -O0, _FORTIFY_SOURCE is disabled and things will work. Alternatively I guess you can compile the whole file with -D_FORTIFY_SOURCE=0, although I haven't tried it.
Also, I could reproduce this on other flavors of linux by just adding -D_FORTIFY_SOURCE=1.
I guess you can call it a compiler/glibc/Ubuntu bug. Or just stop using crazy optimization attributes. The world can't be tested with all possible weird combinations so we should exercise caution when pushing buttons and turning knobs.