diff mbox series

[v2,2/2] read-cache: check range before dereferencing an array element

Message ID 73cae30129338cf219a810c3a2a78ef48d5637d0.1743073557.git.gitgitgadget@gmail.com (mailing list archive)
State New
Headers show
Series Range-check array index before access | expand

Commit Message

Johannes Schindelin March 27, 2025, 11:05 a.m. UTC 
From: Johannes Schindelin <johannes.schindelin@gmx.de>

Before accessing an array element at a given index, we should make sure
that the index is within the desired bounds, otherwise it makes little
sense to access the array element in the first place.

In this instance, testing whether `ce->name[common]` is the trailing NUL
byte is technically different from testing whether `common` is within
the bounds of `previous_name`. It is also redundant, as the range-check
guarantees that `previous_name->buf[common]` cannot be NUL and therefore
the condition `ce->name[common] == previous_name->buf[common]` would not
be met if `ce->name[common]` evaluated to NUL.

However, in the interest of reducing the cognitive load to reason about
the correctness of this loop (so that I can focus on interesting
projects again), I'll simply move the range-check to the beginning of
the loop condition and keep the redundant NUL check.

This acquiesces CodeQL's `cpp/offset-use-before-range-check` rule.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 read-cache.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

Comments

Jeff King March 28, 2025, 5:49 a.m. UTC | #1 
On Thu, Mar 27, 2025 at 11:05:57AM +0000, Johannes Schindelin via GitGitGadget wrote:

> From: Johannes Schindelin <johannes.schindelin@gmx.de>
> 
> Before accessing an array element at a given index, we should make sure
> that the index is within the desired bounds, otherwise it makes little
> sense to access the array element in the first place.
> 
> In this instance, testing whether `ce->name[common]` is the trailing NUL
> byte is technically different from testing whether `common` is within
> the bounds of `previous_name`. It is also redundant, as the range-check
> guarantees that `previous_name->buf[common]` cannot be NUL and therefore
> the condition `ce->name[common] == previous_name->buf[common]` would not
> be met if `ce->name[common]` evaluated to NUL.
> 
> However, in the interest of reducing the cognitive load to reason about
> the correctness of this loop (so that I can focus on interesting
> projects again), I'll simply move the range-check to the beginning of
> the loop condition and keep the redundant NUL check.

Thanks, I think this explanation works, and the patch looks fine. (I
didn't dig deeply into patch 1, but I agree with Junio's analysis that
it is also a false positive).

-Peff
diff mbox series

Patch

diff --git a/read-cache.c b/read-cache.c
index e678c13e8f1..08ae66ad609 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -2686,8 +2686,8 @@  static int ce_write_entry(struct hashfile *f, struct cache_entry *ce,
 		int common, to_remove, prefix_size;
 		unsigned char to_remove_vi[16];
 		for (common = 0;
-		     (ce->name[common] &&
-		      common < previous_name->len &&
+		     (common < previous_name->len &&
+		      ce->name[common] &&
 		      ce->name[common] == previous_name->buf[common]);
 		     common++)
 			; /* still matching */