Can't execute system call "execve"

Question

I am a newbie so I could be inacurate with expressions. I need to make a system call "execve" in C with assembler. I don't use any libraries. Part that doesn't work is

char *nul=(char *)0;
char *argv[] = { "/bin/date", nul };
char *envp[] = { nul };
long ret;
asm volatile ("int $0x80" : "=a" (ret) : "a" (11), "b" (argv[0]), "c" (argv), "d" (envp));
//"a" (11) ... 11 correspondes to execve

I compile the code (and get neither errors nor warnings) with

gcc -m32 -nostdlib -nostdinc -static -O2 sysc.c -o sysc

When I try to run the program I see this message:

A NULL argv[0] was passed through an exec system call.
Aborted

Why do you need to use assembler to call a C function from a C program? (I'm not saying there's no possible reason, but knowing your goal might help in answering the question.) — Keith Thompson, Apr 29 '14 at 23:09
`c` should also be `argv[0]`. Pointers to arrays are incredibly rare and useless in C; typically you always operate on the pointer to the first *element*. — Kerrek SB, Apr 29 '14 at 23:28
@RomanSushkov: It wasn't meant to help, just to fix the types in your call :-) — Kerrek SB, Apr 30 '14 at 08:57
Possible duplicate of [Why can the execve system call run "/bin/sh" without any argv arguments, but not "/bin/ls"?](http://stackoverflow.com/questions/36673765/why-can-the-execve-system-call-run-bin-sh-without-any-argv-arguments-but-not) — Ciro Santilli OurBigBook.com, Feb 17 '17 at 07:06
Before closing as a duplicate, can anyone confirm (1) ASM does not affect the issue; and (2) `/bin/date` falls under coreutil's umbrella and exhibits the same behavior as `/bin/ls`. — jww, Feb 19 '17 at 21:06

score 1 · Accepted Answer · answered Apr 29 '14 at 23:18

If you look at the generated code, you'll see that the compiler optimized away the initialization of argv and envp, on the assumption that your asm block is not accessing them (since you only declare that you need the pointers themselves).

Solution: add a "memory" clobber to tell the compiler that you may read or write any memory.

score 0 · Answer 2 · answered Apr 29 '14 at 23:22

0

The arguments to the exec() family of functions are a bit strange.

In particular, arg0 and arg1 are roughly same thing:

char *args [] = {"./path/to/program", "arg1", "arg2", NULL};

int rc = execve (args[0], args, envp);

answered Apr 29 '14 at 23:22

wallyk

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Can't execute system call "execve"

2 Answers2