[PATCH v2] tools/nolibc: avoid error in dup2() if old fd equals new fd

[PATCH v2] tools/nolibc: avoid error in dup2() if old fd equals new fd

[Thomas Weißschuh]

dup2() allows both 'old' and 'new' to have the same value, which dup3()
does not. If libc dup2() is implemented through the dup3() system call,
then it would incorrectly fail in this case.

Avoid the error by handling old == new explicitly.

Fixes: 30ca20517ac1 ("tools headers: Move the nolibc header from rcutorture to tools/include/nolibc/")
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
---
Changes in v2:
- Correctly handle the case where 'old' is invalid
- Link to v1: https://lore.kernel.org/r/20250819-nolibc-dup2-einval-v1-1-d6c8308cdc50@linutronix.de
---
 tools/include/nolibc/sys.h | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/tools/include/nolibc/sys.h b/tools/include/nolibc/sys.h
index 295e71d34abadb7f9c7ca995012b4395b6830975..8aed6c5f00a9186b943e388f9d5f702412c77dd2 100644
--- a/tools/include/nolibc/sys.h
+++ b/tools/include/nolibc/sys.h
@@ -238,6 +238,13 @@ static __attribute__((unused))
 int sys_dup2(int old, int new)
 {
 #if defined(__NR_dup3)
+	int ret;
+
+	if (old == new) {
+		ret = my_syscall2(__NR_fcntl, old, F_GETFD);
+		return ret < 0 ? ret : old;
+	}
+
 	return my_syscall3(__NR_dup3, old, new, 0);
 #elif defined(__NR_dup2)
 	return my_syscall2(__NR_dup2, old, new);

---
base-commit: 8f5ae30d69d7543eee0d70083daf4de8fe15d585
change-id: 20250819-nolibc-dup2-einval-825bfd77de7a

Best regards,
-- 
Thomas Weißschuh <thomas.weissschuh@linutronix.de>

Re: [PATCH v2] tools/nolibc: avoid error in dup2() if old fd equals new fd

[Willy Tarreau]

On Wed, Aug 20, 2025 at 10:29:27AM +0200, Thomas Weißschuh wrote:
> dup2() allows both 'old' and 'new' to have the same value, which dup3()
> does not. If libc dup2() is implemented through the dup3() system call,
> then it would incorrectly fail in this case.
> 
> Avoid the error by handling old == new explicitly.
> 
> Fixes: 30ca20517ac1 ("tools headers: Move the nolibc header from rcutorture to tools/include/nolibc/")
> Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>

Good, LGTM!

Acked-by: Willy Tarreau <w@1wt.eu>

Willy