Using mprotect to make text segment writable on macOS

Question

This is essentially what I'm trying to do,

#include <sys/mman.h>

int zero() {
    return 0;
}

int main(int argc, const char *argv[]) {
    return mprotect((void *) &zero, 4096, PROT_READ | PROT_WRITE);
}

so I'm trying to make code writable, essentially. This doesn't work on the current macOS (Catalina 10.15.2), it just returns -1 and sets errno to EACCES, which as far as I know is because of lack of entitlement/code signing. I've found the entitlement I need to set, but I have no idea how to go about that, nor how to actually sign it..

If I run codesign -d --entitlements :- <path_to_app>, it fails with code object is not signed at all, even though I've tried configuring signing in Xcode for a while (I have a certificate and so on). So how should I go about this? Actually signing it isn't obvious with Xcode, so I'm fairly clueless.

Answer 1

This is not a definitive answer, but it's a workaround.

Your problem is caused by changes of the linker (ld64) in macOS Catalina. The default value of the max_prot attribute of the __TEXT segment in the Mach-O header has been changed.

Previously max_prot default value was 0x7 (PROT_READ | PROT_WRITE | PROT_EXEC).
The default value has now been changed to 0x5 (PROT_READ | PROT_EXEC).

This means that mprotect cannot make any region that resides within __TEXT writable.

In theory, this should be resolved by providing the linker flag -segprot __TEXT rwx rx, but this is not the case. Since Catalina, the max_prot field is ignored. Instead, max_prot is set to the value of init_prot (see here).

To top it all off, init_prot cannot be set to rwx either due to macOS refusing to execute a file which has a writable __TEXT(init_prot) attribute.

A brute workaround is to manually modify and set __TEXT(max_prot) to 0x7 after linking.

printf '\x07' | dd of=<executable> bs=1 seek=160 count=1 conv=notrunc

Since that code snippet relies on the __TEXT(max_prot) offset being hardcoded to 0xA0, as an alternative, I've created a drop-in replacement/wrapper for ld which respects the max_prot parameter of segprot.