[bluez/bluez] b0c2f7: monitor: clamp max_len in print_packet

[Chad Spensky <noreply@github.com>]

  Branch: refs/heads/1146334
  Home:   https://github.com/bluez/bluez
  Commit: b0c2f7dfc2a32a8479e9663db6e440795dc4b702
      https://github.com/bluez/bluez/commit/b0c2f7dfc2a32a8479e9663db6e440795dc4b702
  Author: Chad Spensky <chad@allthenticate.com>
  Date:   2026-08-14 (Fri, 14 Aug 2026)

  Changed paths:
    M monitor/packet.c

  Log Message:
  -----------
  monitor: clamp max_len in print_packet

print_packet() derives the snprintf() bound for the packet text from the
terminal width and never bounds it against the space left in line[]:

	char line[LINE_MAX], ts_str[96], pid_str[140];
	int col = num_columns();
	...
	int max_len = col - len - extra_len - ts_len - 3;

	if (max_len <= 0) {
		extra = NULL;
		max_len = col - len - ts_len - 3;
	}

	n = snprintf(line + pos, max_len + 1, "%s%s",
					label ? ": " : "", text);

max_len can leave range in both directions, and either aborts under
_FORTIFY_SOURCE with "*** buffer overflow detected ***: terminated":

 - The existing max_len <= 0 recovery drops extra and recomputes, but if
   the prefix alone exceeds the column budget the result is still
   negative, and max_len + 1 then underflows when converted to size_t.
   len includes the "comm[pid]: " prefix built from struct ucred, which
   is only present when reading from the monitor socket, so this is
   reachable at ordinary widths. On a host with a large kernel.pid_max
   the pid is 7 digits, so a long process name plus a long label is
   enough and btmon dies mid-capture. Replaying the same traffic from a
   btsnoop file never reproduces it, because there is no ucred and hence
   no prefix.

 - col larger than sizeof(line) makes max_len + 1 exceed the remaining
   buffer. LINE_MAX raised the bar but did not remove it.

A negative max_len is also used to index line[] when truncating the
text, writing before the start of the buffer, so this is an
out-of-bounds write and not only an abort.

The check only triggers at _FORTIFY_SOURCE=3; at =2 the runtime pos
offset defeats __builtin_object_size and the check is elided, which is
why this is mostly seen on distributions defaulting to =3.

Clamp max_len to the space remaining in line[], and only write the
ellipsis when there is room for it.

Reproduced and verified by building the current logic and the patched
logic with -O2 -D_FORTIFY_SOURCE=3 and replaying a capture at a range of
terminal widths:

	cols   before   after
	  10   abort    ok
	  20   abort    ok
	  40   abort    ok
	  80   ok       ok
	 200   ok       ok
	2000   ok       ok
	3000   abort    ok
	5000   abort    ok

Reported-at: https://github.com/bluez/bluez/issues/1104



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