[PATCH 1/5] archive-tar: guard get_path_prefix against zero-length input

["Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>]

From: Johannes Schindelin <johannes.schindelin@gmx.de>

The `get_path_prefix()` function uses a `do ... while()` loop that
unconditionally decrements the `size_t` index variable before checking
the loop condition. When that variable is 0 on entry, the decrement
wraps to `SIZE_MAX`, and `path[SIZE_MAX]` is an out-of-bounds read.

The current caller (`write_tar_entry()`) only invokes this function when
`pathlen > sizeof(header.name)` i.e. when `pathlen` is at least 100.
Therefore, the bug is unreachable in practice.

However, the function's interface accepts arbitrary pathlen and maxlen
values, and a future caller or a maxlen of 0 could trigger the wrap, so
let's add a check for that.

Pointed out by Coverity.

Assisted-by: Claude Opus 4.6
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 archive-tar.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/archive-tar.c b/archive-tar.c
index a1c66024d4..2da6e55484 100644
--- a/archive-tar.c
+++ b/archive-tar.c
@@ -210,6 +210,8 @@ static size_t get_path_prefix(const char *path, size_t pathlen, size_t maxlen)
 		i--;
 	if (i > maxlen)
 		i = maxlen;
+	if (!i)
+		return 0;
 	do {
 		i--;
 	} while (i > 0 && path[i] != '/');
-- 
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