[libc-commits] [libc] [libc] Reject unsupported flags in fchmodat with ENOTSUP (PR #223165)

[Aman Maurya via libc-commits]

https://github.com/amanmaurya92 updated https://github.com/llvm/llvm-project/pull/223165

>From 740a8663b80dfb273f68d6acbb288dae6262cedc Mon Sep 17 00:00:00 2001
From: amanmaurya92 <amanmaurya9209 at gmail.com>
Date: Sat, 12 Sep 2026 23:41:12 +0530
Subject: [PATCH] [libc] Reject unsupported flags in fchmodat with ENOTSUP

Pass 3 arguments to SYS_fchmodat and reject non-zero flags with ENOTSUP in the fchmodat entrypoint.

Fixes #223072

Assisted-by: LLM
---
 .../__support/OSUtil/linux/syscall_wrappers/fchmodat.h |  5 ++---
 libc/src/sys/stat/linux/CMakeLists.txt                 |  1 +
 libc/src/sys/stat/linux/fchmodat.cpp                   |  7 ++++++-
 libc/test/src/sys/stat/CMakeLists.txt                  |  1 +
 libc/test/src/sys/stat/fchmodat_test.cpp               | 10 ++++++++++
 5 files changed, 20 insertions(+), 4 deletions(-)

diff --git a/libc/src/__support/OSUtil/linux/syscall_wrappers/fchmodat.h b/libc/src/__support/OSUtil/linux/syscall_wrappers/fchmodat.h
index 7b8ee1a079b99..b62a040e9fe54 100644
--- a/libc/src/__support/OSUtil/linux/syscall_wrappers/fchmodat.h
+++ b/libc/src/__support/OSUtil/linux/syscall_wrappers/fchmodat.h
@@ -24,9 +24,8 @@
 namespace LIBC_NAMESPACE_DECL {
 namespace linux_syscalls {
 
-LIBC_INLINE ErrorOr<int> fchmodat(int fd, const char *path, mode_t mode,
-                                  int flags) {
-  int ret = syscall_impl<int>(SYS_fchmodat, fd, path, mode, flags);
+LIBC_INLINE ErrorOr<int> fchmodat(int fd, const char *path, mode_t mode) {
+  int ret = syscall_impl<int>(SYS_fchmodat, fd, path, mode);
   if (ret < 0)
     return Error(-ret);
   return ret;
diff --git a/libc/src/sys/stat/linux/CMakeLists.txt b/libc/src/sys/stat/linux/CMakeLists.txt
index 52ae4566a2b87..953e18be159d8 100644
--- a/libc/src/sys/stat/linux/CMakeLists.txt
+++ b/libc/src/sys/stat/linux/CMakeLists.txt
@@ -30,6 +30,7 @@ add_entrypoint_object(
   HDRS
     ../fchmodat.h
   DEPENDS
+    libc.hdr.errno_macros
     libc.hdr.types.mode_t
     libc.src.__support.OSUtil.linux.syscall_wrappers.fchmodat
     libc.src.errno.errno
diff --git a/libc/src/sys/stat/linux/fchmodat.cpp b/libc/src/sys/stat/linux/fchmodat.cpp
index 38e02b5d61617..29bee319ed0ad 100644
--- a/libc/src/sys/stat/linux/fchmodat.cpp
+++ b/libc/src/sys/stat/linux/fchmodat.cpp
@@ -8,6 +8,7 @@
 
 #include "src/sys/stat/fchmodat.h"
 
+#include "hdr/errno_macros.h"
 #include "src/__support/OSUtil/linux/syscall_wrappers/fchmodat.h"
 #include "src/__support/common.h"
 #include "src/__support/libc_errno.h"
@@ -17,7 +18,11 @@ namespace LIBC_NAMESPACE_DECL {
 
 LLVM_LIBC_FUNCTION(int, fchmodat,
                    (int dirfd, const char *path, mode_t mode, int flags)) {
-  auto result = linux_syscalls::fchmodat(dirfd, path, mode, flags);
+  if (flags != 0) {
+    libc_errno = ENOTSUP;
+    return -1;
+  }
+  auto result = linux_syscalls::fchmodat(dirfd, path, mode);
   if (!result) {
     libc_errno = result.error();
     return -1;
diff --git a/libc/test/src/sys/stat/CMakeLists.txt b/libc/test/src/sys/stat/CMakeLists.txt
index 3e87a11b05e6e..ec083d5372971 100644
--- a/libc/test/src/sys/stat/CMakeLists.txt
+++ b/libc/test/src/sys/stat/CMakeLists.txt
@@ -28,6 +28,7 @@ add_libc_test(
   SRCS
     fchmodat_test.cpp
   DEPENDS
+    libc.hdr.errno_macros
     libc.hdr.fcntl_macros
     libc.hdr.sys_stat_macros
     libc.hdr.types.struct_stat
diff --git a/libc/test/src/sys/stat/fchmodat_test.cpp b/libc/test/src/sys/stat/fchmodat_test.cpp
index 8a1a9379aca71..868cdb2bc817a 100644
--- a/libc/test/src/sys/stat/fchmodat_test.cpp
+++ b/libc/test/src/sys/stat/fchmodat_test.cpp
@@ -6,6 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "hdr/errno_macros.h"
 #include "hdr/fcntl_macros.h"
 #include "hdr/sys_stat_macros.h"
 #include "hdr/types/struct_stat.h"
@@ -65,3 +66,12 @@ TEST_F(LlvmLibcFchmodatTest, NonExistentFile) {
       LIBC_NAMESPACE::fchmodat(AT_FDCWD, "non-existent-file", S_IRUSR, 0),
       Fails(ENOENT));
 }
+
+TEST_F(LlvmLibcFchmodatTest, UnsupportedFlags) {
+  ASSERT_THAT(LIBC_NAMESPACE::fchmodat(AT_FDCWD, "non-existent-file", S_IRUSR,
+                                       AT_SYMLINK_NOFOLLOW),
+              Fails(ENOTSUP));
+  ASSERT_THAT(
+      LIBC_NAMESPACE::fchmodat(AT_FDCWD, "non-existent-file", S_IRUSR, -1),
+      Fails(ENOTSUP));
+}