Reflect application changes to argv[0] in /proc/self/cmdline

[wcampbell-nv]

This is a simple fix for issue #5069, adjusting /proc/self/cmdline emulation for ELFCodeLoader to output the contents of the stack's argv data rather than the stashed argument strings. This means application writes to argv[0] will be visible to in-process /proc/self/cmdline reads. The change is still not visible to external processes.

I've tested that the rename_process.c script attached to the issue now works the same in and out of FEX:

➜ gcc rename_process.c
➜ ./a.out
/proc/self/cmdline: ./a.out
Rewrote argv[0] to RENAMED
/proc/self/cmdline: RENAMED

➜ x86_64-linux-gnu-gcc ./rename_process.c
➜ FEXInterpreter ./a.out
/proc/self/cmdline: ./a.out
Rewrote argv[0] to RENAMED
/proc/self/cmdline: RENAMED

[Sonicadvance1]

Oh wow, I completely missed that would have been a possible work around. Good job!

[wcampbell-nv]

I wouldn't have known this was possible without your comment :)

I see one test is failing, but it looks unrelated to my change. Could this be a test issue?

[Sonicadvance1]

That's definitely a spooky spurious failure.

[neobrain]

fextl::string is null-terminated:

Suggested change

	const auto& Args = GetApplicationArguments();
	const char NullChar {};
	// cmdline is an array of null terminated arguments
	for (const auto& Arg : Args) {
	write(fd, Arg.c_str(), Arg.size());
	write(fd, &NullChar, sizeof(uint8_t));
	}
	const auto& Args = GetApplicationArguments();
	// cmdline is an array of null terminated arguments
	for (const auto& Arg : Args) {
	write(fd, Arg.c_str(), Arg.size() + 1);
	}

Btw is this implementation used anywhere in practice or do we only ever call the override from ELFCodeLoader.h?

[neobrain]

This line probably merits an explanatory comment.

[neobrain]

The change is still not visible to external processes

Have you tried using the prctl(PR_SET_MM, PR_SET_MM_MAP, ...) approach that was suggested in the issue? Seems like that would allow us to drop the /proc/self/cmdline hook entirely even.