[Bug analyzer/106229] New: False positives from -Wanalyzer-tainted-array-index with unsigned char index

["dmalcolm at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106229

            Bug ID: 106229
           Summary: False positives from -Wanalyzer-tainted-array-index
                    with unsigned char index
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: analyzer
          Assignee: dmalcolm at gcc dot gnu.org
          Reporter: dmalcolm at gcc dot gnu.org
  Target Milestone: ---

Consider:

struct s_12
{
  unsigned char idx;
  char buf[256];
};

char __attribute__((tainted_args))
test_12(struct s_12 s)
{
  return s.buf[s.idx];
}

Currently with trunk and gcc 12 this gives:

<source>: In function 'test_12':
<source>:10:15: warning: use of attacker-controlled value 's.idx' in array
lookup without bounds checking [CWE-129] [-Wanalyzer-tainted-array-index]
   10 |   return s.buf[s.idx];
      |          ~~~~~^~~~~~~
  'test_12': event 1
    |
    |    8 | test_12(struct s_12 s)
    |      | ^~~~~~~
    |      | |
    |      | (1) function 'test_12' marked with '__attribute__((tainted_args))'
    |
    +--> 'test_12': events 2-3
           |
           |    8 | test_12(struct s_12 s)
           |      | ^~~~~~~
           |      | |
           |      | (2) entry to 'test_12'
           |    9 | {
           |   10 |   return s.buf[s.idx];
           |      |          ~~~~~~~~~~~~
           |      |               |
           |      |               (3) use of attacker-controlled value 's.idx'
in array lookup without bounds checking

https://godbolt.org/z/ozhWdb78G

However, given that s.idx is unsigned char, it must be within the valid range,
and so the warning is unhelpful.

See on Linux kernel in drivers/tty/vt/keyboard.c where ioctls use a
user-supplied index to access the key_maps array:

include/linux/keyboard.h:extern unsigned short *key_maps[MAX_NR_KEYMAPS];
include/uapi/linux/keyboard.h:#define MAX_NR_KEYMAPS    256

but the index is unsigned char, so must be within range.