diff: fix out-of-bounds reads and NULL deref in diffstat UTF-8 truncation

f85b49f3d4a (diff: improve scaling of filenames in diffstat to handle
UTF-8 chars, 2026-01-16) introduced a loop in show_stats() that calls
utf8_width() repeatedly to skip leading characters until the displayed
width fits.  However, utf8_width() can return problematic values:

  - For invalid UTF-8 sequences, pick_one_utf8_char() sets the name
    pointer to NULL and utf8_width() returns 0.  Since name_len does
    not change, the loop iterates once more and pick_one_utf8_char()
    dereferences the NULL pointer, crashing.

  - For control characters, utf8_width() returns -1, so name_len
    grows when it is expected to shrink.  This can cause the loop to
    consume more characters than the string contains, reading past
    the trailing NUL.

By default, fill_print_name() will C-quotes filenames which escapes
control characters and invalid bytes to printable text.  That avoids
this bug from being triggered; however, with core.quotePath=false,
raw bytes can reach this code.

Add tests exercising both failure modes with core.quotePath=false and
a narrow --stat-name-width to force truncation: one with a bare 0xC0
byte (invalid UTF-8 lead byte, triggers NULL deref) and one with a
0x01 byte (control character, causes the loop to read past the end
of the string).

Fix both issues by introducing utf8_ish_width(), a thin wrapper
around utf8_width() that guarantees the pointer always advances and
the returned width is never negative:

  - On invalid UTF-8 it restores the pointer, advances by one byte,
    and returns width 1 (matching the strlen()-based fallback used
    by utf8_strwidth()).
  - On a control character it returns 0 (matching utf8_strnwidth()
    which skips them).

Also add a "&& *name" guard to the while-loop condition so it
terminates at end-of-string even when utf8_strwidth()'s strlen()
fallback causes name_len to exceed the sum of per-character widths.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

0 files changed, 0 insertions, 0 deletions