[cctools] fix ar(1) crash without argument, closes #7.

[biergaizi]

In ar(1), strcmp() checks are used to determine the value of argument argv[1], even when no argument is given. In the past, they were possibly harmless out-of-bound reads and comparison with garbage, without consequences.

However, running it on macOS 13 w/ Apple Silicon immediately crashes it with Segmentation Fault, because argv[1] is now NULL and generates EXC_BAD_ACCESS in strcmp().

This commit checks whether argc is equal or greater than 2 before doing strcmp() to resolve Issue #7.

$ ./bin/ar
Segmentation fault: 11

* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x0) frame #0: 0x0000000193181460 libsystem_platform.dylib`_platform_strcmp + 144 libsystem_platform.dylib`:
->  0x193181460 <+144>: ldr    q0, [x0], #0x10
    0x193181464 <+148>: ldr    q1, [x1], #0x10
    0x193181468 <+152>: cmeq.16b v1, v0, v1
    0x19318146c <+156>: and.16b v0, v0, v1
Target 0: (ar) stopped.
(lldb) bt
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x0)
  * frame #0: 0x0000000193181460 libsystem_platform.dylib`_platform_strcmp + 144
    frame #1: 0x0000000100006440 ar`main(argc=1, argv=0x000000016fdfef58) at ar.c:126:8
    frame #2: 0x0000000192e2be50 dyld`start + 2544

[cooljeanius]

While you're in the area, I wonder if it's worthwhile to change the calls to strcmp to strncmp as well?

[biergaizi]

I wonder if it's worthwhile to change the calls to strcmp to strncmp as well?

I don't think it's worthwhile. While strcmp() in general should be avoided, in thin particular case, the strings in **argv from the system are always valid, as long as both string inputs are not NULL to strcmp(), it's perfectly safe.

[cooljeanius]

I think this is also in PR #4 but that's ok; it's probably better to get this one in first since it's smaller and stand-alone

[cooljeanius]

I wonder if it's worthwhile to change the calls to strcmp to strncmp as well?

I don't think it's worthwhile. While strcmp() in general should be avoided, in thin particular case, the strings in **argv from the system are always valid, as long as both string inputs are not NULL to strcmp(), it's perfectly safe.

Hmmm @iains WDYT?

[iains]

in some cases, I'd agree that changing to strncmp makes sense (e.g. if we allocated the space and we know what it is). However in the case of the strings passed as argv[], the OS allocated the space, we have no idea what the correct 'n' would be - we have to trust that the OS makes a valid process launch (absent any mechanism to determine something different).

so I think we're fine here to just check for missing argv[]s