[PATCH 3/5] strbuf_readlink(): avoid calling `readlink()` twice in corner-cases

From: Karsten Blees via GitGitGadget <gitgitgadget@gmail.com>
Date: Dec 16, 2025, 15:33 UTC
Message-ID: <dba281027a8fc49fc152a8d072761ea238aa54d4.1765899229.git.gitgitgadget@gmail.com>
In-Reply-To: <pull.2017.git.1765899229.gitgitgadget@gmail.com>

From: Karsten Blees <blees@dcon.de>

The `strbuf_readlink()` function calls `readlink()`` twice if the hint argument specifies the exact size of the link target (e.g. by passing stat.st_size as returned by `lstat()`). This is necessary because `readlink(..., hint) == hint` could mean that the buffer was too small.

Use `hint + 1` as buffer size to prevent this.

Signed-off-by: Karsten Blees <blees@dcon.de>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 strbuf.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/strbuf.c b/strbuf.c
index 6c3851a7f8..44a8f6a554 100644
--- a/strbuf.c
+++ b/strbuf.c
@@ -578,12 +578,12 @@ int strbuf_readlink(struct strbuf *sb, const char *path, size_t hint)
 	while (hint < STRBUF_MAXLINK) {
 		ssize_t len;
 
-		strbuf_grow(sb, hint);
-		len = readlink(path, sb->buf, hint);
+		strbuf_grow(sb, hint + 1);
+		len = readlink(path, sb->buf, hint + 1);
 		if (len < 0) {
 			if (errno != ERANGE)
 				break;
-		} else if (len < hint) {
+		} else if (len <= hint) {
 			strbuf_setlen(sb, len);
 			return 0;
 		}

-- 
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