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compat: helper for detecting unsigned overflow

The idiom (a + b < a) works fine for detecting that an unsigned
integer has overflowed, but a more explicit

	unsigned_add_overflows(a, b)

might be easier to read.

Define such a macro, expanding roughly to ((a) < UINT_MAX - (b)).
Because the expansion uses each argument only once outside of sizeof()
expressions, it is safe to use with arguments that have side effects.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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commit 1368f65002bf39fdde7dd736a75ae35475184371 1 parent a8e4a59
@jrn jrn authored committed
Showing with 11 additions and 4 deletions.
  1. +6 −0 git-compat-util.h
  2. +1 −1  patch-delta.c
  3. +3 −2 strbuf.c
  4. +1 −1  wrapper.c
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6 git-compat-util.h
@@ -31,6 +31,9 @@
#define maximum_signed_value_of_type(a) \
(INTMAX_MAX >> (bitsizeof(intmax_t) - bitsizeof(a)))
+#define maximum_unsigned_value_of_type(a) \
+ (UINTMAX_MAX >> (bitsizeof(uintmax_t) - bitsizeof(a)))
+
/*
* Signed integer overflow is undefined in C, so here's a helper macro
* to detect if the sum of two integers will overflow.
@@ -40,6 +43,9 @@
#define signed_add_overflows(a, b) \
((b) > maximum_signed_value_of_type(a) - (a))
+#define unsigned_add_overflows(a, b) \
+ ((b) > maximum_unsigned_value_of_type(a) - (a))
+
#ifdef __GNUC__
#define TYPEOF(x) (__typeof__(x))
#else
View
2  patch-delta.c
@@ -48,7 +48,7 @@ void *patch_delta(const void *src_buf, unsigned long src_size,
if (cmd & 0x20) cp_size |= (*data++ << 8);
if (cmd & 0x40) cp_size |= (*data++ << 16);
if (cp_size == 0) cp_size = 0x10000;
- if (cp_off + cp_size < cp_size ||
+ if (unsigned_add_overflows(cp_off, cp_size) ||
cp_off + cp_size > src_size ||
cp_size > size)
break;
View
5 strbuf.c
@@ -63,7 +63,8 @@ void strbuf_attach(struct strbuf *sb, void *buf, size_t len, size_t alloc)
void strbuf_grow(struct strbuf *sb, size_t extra)
{
- if (sb->len + extra + 1 <= sb->len)
+ if (unsigned_add_overflows(extra, 1) ||
+ unsigned_add_overflows(sb->len, extra + 1))
die("you want to use way too much memory");
if (!sb->alloc)
sb->buf = NULL;
@@ -152,7 +153,7 @@ int strbuf_cmp(const struct strbuf *a, const struct strbuf *b)
void strbuf_splice(struct strbuf *sb, size_t pos, size_t len,
const void *data, size_t dlen)
{
- if (pos + len < pos)
+ if (unsigned_add_overflows(pos, len))
die("you want to use way too much memory");
if (pos > sb->len)
die("`pos' is too far after the end of the buffer");
View
2  wrapper.c
@@ -53,7 +53,7 @@ void *xmalloc(size_t size)
void *xmallocz(size_t size)
{
void *ret;
- if (size + 1 < size)
+ if (unsigned_add_overflows(size, 1))
die("Data too large to fit into virtual memory space.");
ret = xmalloc(size + 1);
((char*)ret)[size] = 0;
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