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Unexpected exec permission from mmap when assembly files included in the project (2)

As an alternative to modifying your assembly files with GNU-specific section directive variants, you can add -Wa,--noexecstack to your command line for building assembly files. For example, see how I do it in musl's configure :

https://git.musl-libc.org/cgit/musl/commit/configure?id=adefe830dd376be386df5650a09c313c483adf1a

I believe at least some versions of clang with integrated-assembler may require it to be passed as --noexecstack (without the -Wa ), so your configure script should probably check both and see which is accepted.

You can also use -Wl,-z,noexecstack at link time (in LDFLAGS ) to get the same result. The disadvantage of this is that it doesn't help if your project produces static ( .a ) library files for use by other software, since you then don't control the link-time options when it's used by other programs.

I am banging my head into the wall with this.

In my project, when I'm allocating memory with mmap the mapping ( /proc/self/maps ) shows that it is an readable and executable region despite I requested only readable memory.

After looking into strace (which was looking good) and other debugging, I was able to identify the only thing that seems to avoid this strange problem: removing assembly files from the project and leaving only pure C. (what?!)

So here is my strange example, I am working on Ubunbtu 19.04 and default gcc.

If you compile the target executable with the ASM file (which is empty) then mmap returns a readable and executable region, if you build without then it behave correctly. See the output of /proc/self/maps which I have embedded in my example.

example.c

#include <stdio.h>
#include <string.h>
#include <sys/mman.h>

int main()
{
    void* p;
    p = mmap(NULL, 8192,PROT_READ,MAP_ANONYMOUS|MAP_PRIVATE,-1,0);

    {
        FILE *f;
        char line[512], s_search[17];
        snprintf(s_search,16,"%lx",(long)p);
        f = fopen("/proc/self/maps","r");
        while (fgets(line,512,f))
        {
            if (strstr(line,s_search)) fputs(line,stderr);
        }

        fclose(f);
    }

    return 0;
}

example.s : Is an empty file!

Outputs

With the ASM included version

VirtualBox:~/mechanics/build$ gcc example.c example.s -o example && ./example
7f78d6e08000-7f78d6e0a000 r-xp 00000000 00:00 0 

Without the ASM included version

VirtualBox:~/mechanics/build$ gcc example.c -o example && ./example
7f1569296000-7f1569298000 r--p 00000000 00:00 0 

Linux has an execution domain called READ_IMPLIES_EXEC , which causes all pages allocated with PROT_READ to also be given PROT_EXEC . This program will show you whether that's enabled for itself:

#include <stdio.h>
#include <sys/personality.h>

int main(void) {
    printf("Read-implies-exec is %s\n", personality(0xffffffff) & READ_IMPLIES_EXEC ? "true" : "false");
    return 0;
}

If you compile that along with an empty .s file, you'll see that it's enabled, but without one, it'll be disabled. The initial value of this comes from the ELF meta-information in your binary . Do readelf -Wl example . You'll see this line when you compiled without the empty .s file:

  GNU_STACK      0x000000 0x0000000000000000 0x0000000000000000 0x000000 0x000000 RW  0x10

But this one when you compiled with it:

  GNU_STACK      0x000000 0x0000000000000000 0x0000000000000000 0x000000 0x000000 RWE 0x10

Note RWE instead of just RW . The reason for this is that the linker assumes that your assembly files require read-implies-exec unless it's explicitly told that they don't, and if any part of your program requires read-implies-exec, then it's enabled for your whole program. The assembly files that GCC compiles tell it that it doesn't need this, with this line (you'll see this if you compile with -S ):

        .section        .note.GNU-stack,"",@progbits

Put that line in example.s , and it will serve to tell the linker that it doesn't need it either, and your program will then work as expected.





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