From: René Scharfe Date: Sun, 23 Oct 2016 17:57:30 GMT Subject: Re: [PATCH] hex: use unsigned index for ring buffer Message-ID: In-Reply-To: <20161023091146.p2kmqvgwxdf77dnn@sigill.intra.peff.net> Am 23.10.2016 um 11:11 schrieb Jeff King: > On Sun, Oct 23, 2016 at 11:00:48AM +0200, René Scharfe wrote: > >> Overflow is defined for unsigned integers, but not for signed ones. >> Make the ring buffer index in sha1_to_hex() unsigned to be on the >> safe side. >> >> Signed-off-by: Rene Scharfe >> --- >> Hard to trigger, but probably even harder to diagnose once someone >> somehow manages to do it on some uncommon architecture. > > Indeed. If we are worried about overflow, we would also want to assume > that it wraps at a multiple of 4, but that is probably a sane > assumption. Hmm, I can't think of a way to violate this assumption except with unsigned integers that are only a single bit wide. That would be a weird machine. Are there other possibilities? >> diff --git a/hex.c b/hex.c >> index ab2610e..8c6c189 100644 >> --- a/hex.c >> +++ b/hex.c >> @@ -76,7 +76,7 @@ char *oid_to_hex_r(char *buffer, const struct object_id *oid) >> >> char *sha1_to_hex(const unsigned char *sha1) >> { >> - static int bufno; >> + static unsigned int bufno; >> static char hexbuffer[4][GIT_SHA1_HEXSZ + 1]; >> return sha1_to_hex_r(hexbuffer[3 & ++bufno], sha1); >> } > > I wonder if just truncating bufno would be conceptually simpler (albeit > longer): > > bufno++; > bufno &= 3; > return sha1_to_hex_r(hexbuffer[bufno], sha1); > > You could also write the second line like: > > bufno %= ARRAY_SIZE(hexbuffer); > > which is less magical (right now the set of buffers must be a power of > 2). I expect the compiler could turn that into a bitmask itself. Expelling magic is a good idea. And indeed, at least gcc, clang and icc on x86-64 are smart enough to use an AND instead of dividing (https://godbolt.org/g/rFPpzF). But gcc also adds a sign extension (cltq/cdqe) if we store the truncated value, unlike the other two compilers. I don't see why -- the bit mask operation enforces a value between 0 and 3 (inclusive) and the upper bits of eax are zeroed automatically, so the cltq is effectively a noop. Using size_t gets us rid of the extra instruction and is the right type anyway. It would suffice on its own, hmm.. > I'm fine with any of the options. I guess you'd want a similar patch for > find_unique_abbrev on top of jk/no-looking-at-dotgit-outside-repo. Actually I'd want you to want to amend your series yourself. ;) Maybe I can convince Coccinelle to handle that issue for us. And there's also path.c::get_pathname(). That's enough cases to justify adding a macro, I'd say: --- cache.h | 3 +++ hex.c | 4 ++-- path.c | 4 ++-- 3 files changed, 7 insertions(+), 4 deletions(-) diff --git a/cache.h b/cache.h index 05ecb88..8bb4918 100644 --- a/cache.h +++ b/cache.h @@ -555,6 +555,9 @@ extern int daemonize(void); } \ } while (0) +#define NEXT_RING_ITEM(array, index) \ + (array)[(index) = ((index) + 1) % ARRAY_SIZE(array)] + /* Initialize and use the cache information */ struct lock_file; extern int read_index(struct index_state *); diff --git a/hex.c b/hex.c index ab2610e..5e711b9 100644 --- a/hex.c +++ b/hex.c @@ -76,9 +76,9 @@ char *oid_to_hex_r(char *buffer, const struct object_id *oid) char *sha1_to_hex(const unsigned char *sha1) { - static int bufno; + static size_t bufno; static char hexbuffer[4][GIT_SHA1_HEXSZ + 1]; - return sha1_to_hex_r(hexbuffer[3 & ++bufno], sha1); + return sha1_to_hex_r(NEXT_RING_ITEM(hexbuffer, bufno), sha1); } char *oid_to_hex(const struct object_id *oid) diff --git a/path.c b/path.c index a8e7295..60dba6a 100644 --- a/path.c +++ b/path.c @@ -24,8 +24,8 @@ static struct strbuf *get_pathname(void) static struct strbuf pathname_array[4] = { STRBUF_INIT, STRBUF_INIT, STRBUF_INIT, STRBUF_INIT }; - static int index; - struct strbuf *sb = &pathname_array[3 & ++index]; + static size_t index; + struct strbuf *sb = &NEXT_RING_ITEM(pathname_array, index); strbuf_reset(sb); return sb; } -- 2.10.1