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Message-ID: <20260805075201.3508375-1-matthias.goergens@gmail.com>
Date: Wed, 5 Aug 2026 15:52:00 +0800
From: Matthias Goergens <matthias.goergens@...il.com>
To: musl@...ts.openwall.com
Cc: Matthias Goergens <matthias.goergens@...il.com>
Subject: [PATCH 1/2] stdio: avoid overflow adjusting relative seek offset
For SEEK_CUR, fseeko subtracts the unread input-buffer length from the
caller's offset before invoking the underlying seek operation. A valid
LLONG_MIN offset therefore overflows before the seek can reject the
unrepresentable logical result.
Detect the underflow and fail with EOVERFLOW without flushing or
discarding the stream's buffers.
---
src/stdio/fseek.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)
diff --git a/src/stdio/fseek.c b/src/stdio/fseek.c
index c7425802..9f6a981e 100644
--- a/src/stdio/fseek.c
+++ b/src/stdio/fseek.c
@@ -1,4 +1,5 @@
#include "stdio_impl.h"
+#include <limits.h>
#include <errno.h>
int __fseeko_unlocked(FILE *f, off_t off, int whence)
@@ -10,7 +11,13 @@ int __fseeko_unlocked(FILE *f, off_t off, int whence)
}
/* Adjust relative offset for unread data in buffer, if any. */
- if (whence == SEEK_CUR && f->rend) off -= f->rend - f->rpos;
+ if (whence == SEEK_CUR && f->rend) {
+ if (off < LLONG_MIN + (f->rend - f->rpos)) {
+ errno = EOVERFLOW;
+ return -1;
+ }
+ off -= f->rend - f->rpos;
+ }
/* Flush write buffer, and report error on failure. */
if (f->wpos != f->wbase) {
--
2.55.0
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