NaCl: Fix off-by-one in getcwd() syscall emulation.

sysdeps/nacl/sysdep.h

 #ifndef _NACL_SYSDEP_H
 #define _NACL_SYSDEP_H 1
 
 #if !defined(__ASSEMBLER__)
 #include <futex_emulation.h>
 #include <irt_syscalls.h>
 #include <lowlevellock.h>
 
 /* Implementation of all syscalls for use in platform- and OS- independent code
    as inline functions.  Each function translates the syscall arguments into IRT
@@ skipping 580 matching lines @@
    The system call just returns the length of the buffer filled (which includes
    the ending '\0' character), or zero in case of error.  */
 __extern_always_inline int
 INTERNAL_SYSCALL_getcwd_2 (int *err, char *buf, size_t size)
 {
   int len;
   *err = __nacl_irt_getcwd (buf, size);
   if (*err) {
     return 0;
   }
-  for (len = 0; len < size && buf[len] != '\0'; ++len);
-  return len;
+  for (len = 0; buf[len] != '\0'; ++len);

[Roland McGrath, 2014/12/08 20:56:48]

Why is it safe to presume it's terminated and not check size?
Either way, this should just call strlen or __strnlen rather than using a loop.

[Sam Clegg, 2014/12/08 21:10:29]

Because the __nacl_irt_getcwd will fail if it couldn't fit the enture
null terminiated string in buf.  So we could have returned already
above in this case.

I will try strlen.  I was assuming whoever wrote this has a good reason
for using a loop.

[Sam Clegg, 2014/12/08 22:38:39]

Ok. Done.  Although in the case when __nacl_irt_getcwd violates its contract and
doesn't NULL terminate there isn't much we can do here.. somebody further up the
callstack is going to have hard time.

+  return len + 1;
 }
 
 __extern_always_inline gid_t
 INTERNAL_SYSCALL_getegid_0 (int *err)
 {
   *err = (38 /* ENOSYS */);
   return 0;
 }
 
 __extern_always_inline uid_t
@@ skipping 1642 matching lines @@
 #define PSEUDO_END_ERRVAL(name)                 \
   END (name)
 
 #undef SYSCALL_ERROR_HANDLER_TLS_STORE
 #define SYSCALL_ERROR_HANDLER_TLS_STORE(src, destoff)   \
   movl %gs:0, %eax;                                     \
   movl src, (%eax,destoff)
 
 #endif
 #endif