Re: [PATCH] xdiff: avoid arithmetic overflow in xdl_get_hunk()
From: Junio C Hamano <hidden>
Date: 2025-03-14 22:28:19
René Scharfe [off-list ref] writes:
xdl_get_hunk() calculates the maximum number of common lines between two changes that would fit into the same hunk for the given context options. It involves doubling and addition and thus can overflow if the terms are huge. The type of ctxlen and interhunkctxlen in xdemitconf_t is long, while the type of the corresponding context and interhunkcontext in struct diff_options is int. On many platforms longs are bigger that ints, which prevents the overflow. On Windows they have the same range and the overflow manifests as hunks that are split erroneously and lines being repeated between them. Fix the overflow by checking and not going beyond LONG_MAX. This allows specifying a huge context line count and getting all lines of a changed files in a single hunk, as expected. Reported-by: Jason Cho <redacted> Signed-off-by: René Scharfe <redacted> --- t/t4055-diff-context.sh | 10 ++++++++++ xdiff/xemit.c | 8 +++++++- 2 files changed, 17 insertions(+), 1 deletion(-)
Oh, I love a patch like this that is well thought out to carefully check the bounds, instead of blindly say "ah, counting number of things in size_t solves everything" ;-)
quoted hunk
diff --git a/xdiff/xemit.c b/xdiff/xemit.c index f8e3f25b03..1d40c9cb40 100644 --- a/xdiff/xemit.c +++ b/xdiff/xemit.c@@ -43,6 +43,10 @@ static int xdl_emit_record(xdfile_t *xdf, long ri, char const *pre, xdemitcb_t * return 0; } +static long saturating_add(long a, long b) +{ + return signed_add_overflows(a, b) ? LONG_MAX : a + b; +} /* * Starting at the passed change atom, find the latest change atom to be included@@ -52,7 +56,9 @@ static int xdl_emit_record(xdfile_t *xdf, long ri, char const *pre, xdemitcb_t * xdchange_t *xdl_get_hunk(xdchange_t **xscr, xdemitconf_t const *xecfg) { xdchange_t *xch, *xchp, *lxch; - long max_common = 2 * xecfg->ctxlen + xecfg->interhunkctxlen; + long max_common = saturating_add(saturating_add(xecfg->ctxlen, + xecfg->ctxlen), + xecfg->interhunkctxlen);
Looking good. Thanks. Will queue.