Bug 38212

I've taken a quick look through the assembly dump of __spawni_child function (the piece of code that makes the syscall gets inlined into it); the setresuid syscall (4185) is easy to find, and indeed, I cannot find where $a0 is initialized for it. All the relevant INLINE_SYSCALL seems legit, so this may also be a GCC bug...

Ok, seems like I'm getting to something.

Here is what the relevant piece of code looks like (sysdeps/posix/spawni.c, line 140):

  /* Set the effective user and group IDs.  */
  if ((attr->__flags & POSIX_SPAWN_RESETIDS) != 0
      && (local_seteuid (__getuid ()) != 0
	  || local_setegid (__getgid ())) != 0)
    goto fail;

We are getting EPERM from the local_setuid call.

local_seteuid is a macro that should call setresuid syscall with three arguments: -1, it's argument, and one more -1. For mipsel platform, it's definition from sysdeps/unix/sysv/linux/local-setxid.h, line 8 is used:

# define local_seteuid(id) INLINE_SYSCALL (setresuid, 3, -1, id, -1)

With a few preprocessor tricks, this INLINE_SYSCALL is expanded into a statement expression that is a rather simple wrapper around an asm volatile statement that with a syscall instruction. Preprocessed and reformatted a bit, local_seteuid (__getuid ()) looks like this:


({
     long _sc_err __attribute__ ((unused));
     long result_var = ({
             long _sys_result;
             {
                 register long __s0 asm ("$16") __attribute__ ((unused)) = ((4000 + 185));
                 register long __v0 asm ("$2");
                 register long __a0 asm ("$4") = (long) (-1);
                 register long __a1 asm ("$5") = (long) (__getuid ());
                 register long __a2 asm ("$6") = (long) (-1);
                 register long __a3 asm ("$7");
                 __asm__ volatile (
                         ".set\tnoreorder\n\t"
                         "li\t%0, %2\t\t\t# " "setresuid" "\n\t"
                         "syscall\n\t"
                         ".set\treorder"
                   : "=r" (__v0), "=r" (__a3)
                   : "IK" ((4000 + 185)), "r" (__a0), "r" (__a1), "r" (__a2)
                   : "$1", "$3", "$8", "$9", "$10", "$11", "$12", "$13", "$14", "$15", "$24", "$25", "hi", "lo", "memory"
                   );
                 _sc_err = __a3; _sys_result = __v0;
               }
               _sys_result;
         });
     if ( ((void) (result_var), (long) (_sc_err)) ) {
             (__libc_errno = (((void) (_sc_err), result_var)));
             result_var = -1L;
     }
     result_var;
})

Note that on mips32 the first 4 syscall arguments are put into registers a0-a3 (r4-r7).

The important thing to see in this preprocessed code is that a0 register is initalized before __getuid() is called. Indeed, this is what we can see if we compile the code with -O0 (you'll have to comment out the #error directive in include/libc-symbols.h):

$LBB11 = .
        .loc 2 189 11
        li      $16,4185                        # 0x1059
        li      $4,-1                   # 0xffffffffffffffff
        lui     $2,%hi(__getuid)
        addiu   $2,$2,%lo(__getuid)
        move    $25,$2
        .reloc  1f,R_MIPS_JALR,__getuid
1:      jalr    $25
$LVL11 = .
        lw      $28,16($fp)
        move    $5,$2
        li      $6,-1                   # 0xffffffffffffffff
#APP
 # 189 "../sysdeps/unix/sysv/linux/spawni.c" 1
        .set    noreorder
        li      $2, 4185                        # setresuid
        syscall
        .set    reorder
 # 0 "" 2
#NO_APP

Fist, $4 AKA a0 register is initialized, then __getuid is called -- as a function. And since, according to o32 ABI, a0 register does not have to be preserved across a function call, optimizer can assume that a0 is changed by __getuid, and drop its initialization. And this is exactly what is does. Here's what we get with -O2:

$LBB106 = .
        .loc 1 189 11 view $LVU329
        .loc 1 189 11 view $LVU330
        .loc 1 189 11 view $LVU331
        .loc 1 189 11 view $LVU332
        addiu   $25,$25,%lo(__getuid)
        .reloc  1f,R_MIPS_JALR,__getuid
1:      jalr    $25
$LVL92 = .
        li      $6,-1                   # 0xffffffffffffffff
        lw      $28,16($sp)
        move    $5,$2
        .loc 1 189 11 view $LVU333
        .loc 1 189 11 view $LVU334
        .loc 1 189 11 view $LVU335
#APP
 # 189 "../sysdeps/unix/sysv/linux/spawni.c" 1
        .set    noreorder
        li      $2, 4185                        # setresuid
        syscall
        .set    reorder
 # 0 "" 2
        .loc 1 189 11 view $LVU336
$LVL93 = .
        .loc 1 189 11 view $LVU337
        .loc 1 189 11 is_stmt 0 view $LVU338
#NO_APP

r4 (AKA a0) initialization is simply not there.

Apparently, internal_syscall2, internal_syscall3 and internal_syscall4 macros from sysdeps/unix/sysv/linux/mips/mips32/sysdep.h should be rewritten to evaluate their arguments before initializing the registers.