On Tue, 5 Oct 2010, Erik Faye-Lund wrote:
> Signed integer overflow is not defined in C, so do not depend on it.
>
> This fixes a problem with GCC 4.4.0 and -O3 where the optimizer would
> consider "consumed_bytes > consumed_bytes + bytes" as a constant
> expression, and never execute the die()-call.
>
> Signed-off-by: Erik Faye-Lund <kusmabite@xxxxxxxxx>
I like this. Please also fix the similar case in
builtin/pack-objects.c:write_one().
> ---
> builtin/index-pack.c | 2 +-
> builtin/unpack-objects.c | 2 +-
> git-compat-util.h | 12 ++++++++++++
> 3 files changed, 14 insertions(+), 2 deletions(-)
>
> diff --git a/builtin/index-pack.c b/builtin/index-pack.c
> index 2e680d7..e243d9d 100644
> --- a/builtin/index-pack.c
> +++ b/builtin/index-pack.c
> @@ -161,7 +161,7 @@ static void use(int bytes)
> input_offset += bytes;
>
> /* make sure off_t is sufficiently large not to wrap */
> - if (consumed_bytes > consumed_bytes + bytes)
> + if (signed_add_overflows(consumed_bytes, bytes))
> die("pack too large for current definition of off_t");
> consumed_bytes += bytes;
> }
> diff --git a/builtin/unpack-objects.c b/builtin/unpack-objects.c
> index 685566e..f63973c 100644
> --- a/builtin/unpack-objects.c
> +++ b/builtin/unpack-objects.c
> @@ -83,7 +83,7 @@ static void use(int bytes)
> offset += bytes;
>
> /* make sure off_t is sufficiently large not to wrap */
> - if (consumed_bytes > consumed_bytes + bytes)
> + if (signed_add_overflows(consumed_bytes, bytes))
> die("pack too large for current definition of off_t");
> consumed_bytes += bytes;
> }
> diff --git a/git-compat-util.h b/git-compat-util.h
> index 81883e7..2af8d3e 100644
> --- a/git-compat-util.h
> +++ b/git-compat-util.h
> @@ -28,6 +28,18 @@
> #define ARRAY_SIZE(x) (sizeof(x)/sizeof(x[0]))
> #define bitsizeof(x) (CHAR_BIT * sizeof(x))
>
> +#define maximum_signed_value_of_type(a) \
> + (INTMAX_MAX >> (bitsizeof(intmax_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.
> + *
> + * Requires: a >= 0, typeof(a) equals typeof(b)
> + */
> +#define signed_add_overflows(a, b) \
> + ((b) > maximum_signed_value_of_type(a) - (a))
> +
> #ifdef __GNUC__
> #define TYPEOF(x) (__typeof__(x))
> #else
> --
> 1.7.3.1.51.g3f36d.dirty
>
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