[libc] Implement faccessat (llvm#161065)

llvm#160404

- Implement POSIX function "faccessat"
- Remove redundant param in facessat syscall in access implementation,
faccessat syscall does not take a flags arg

Author: mleleszi

libc/config/linux/aarch64/entrypoints.txt

@@ -325,6 +325,7 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.unistd.dup2
     libc.src.unistd.dup3
     libc.src.unistd.execve
+    libc.src.unistd.faccessat
     libc.src.unistd.fchdir
     libc.src.unistd.fpathconf
     libc.src.unistd.fsync

libc/config/linux/x86_64/entrypoints.txt

@@ -331,6 +331,7 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.unistd.dup2
     libc.src.unistd.dup3
     libc.src.unistd.execve
+    libc.src.unistd.faccessat
     libc.src.unistd.fchdir
     libc.src.unistd.fpathconf
     libc.src.unistd.fsync

libc/include/llvm-libc-macros/linux/fcntl-macros.h

@@ -61,6 +61,9 @@
 // Allow empty relative pathname.
 #define AT_EMPTY_PATH 0x1000
 
+// Perform access checks using the effective user and group IDs.
+#define AT_EACCESS 0x200
+
 // Values of SYS_fcntl commands.
 #define F_DUPFD 0
 #define F_GETFD 1

libc/include/sys/syscall.h.def

@@ -309,6 +309,10 @@
 #define SYS_faccessat __NR_faccessat
 #endif
 
+#ifdef __NR_faccessat2
+#define SYS_faccessat2 __NR_faccessat2
+#endif
+
 #ifdef __NR_fadvise64
 #define SYS_fadvise64 __NR_fadvise64
 #endif

libc/include/unistd.yaml

@@ -96,6 +96,15 @@ functions:
       - type: const char *
       - type: __exec_argv_t
       - type: __exec_envp_t
+  - name: faccessat
+    standards:
+      - POSIX
+    return_type: int
+    arguments:
+      - type: int
+      - type: const char *
+      - type: int
+      - type: int
   - name: fchdir
     standards:
       - POSIX

libc/src/unistd/CMakeLists.txt

@@ -55,6 +55,13 @@ add_entrypoint_object(
     .${LIBC_TARGET_OS}.dup3
 )
 
+add_entrypoint_object(
+  faccessat
+  ALIAS
+  DEPENDS
+    .${LIBC_TARGET_OS}.faccessat
+)
+
 add_entrypoint_object(
   fchdir
   ALIAS

libc/src/unistd/faccessat.h

@@ -0,0 +1,20 @@
+//===-- Implementation header for faccessat ---------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_UNISTD_FACCESSAT_H
+#define LLVM_LIBC_SRC_UNISTD_FACCESSAT_H
+
+#include "src/__support/macros/config.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+int faccessat(int fd, const char *path, int amode, int flag);
+
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC_UNISTD_FACCESSAT_H

libc/src/unistd/linux/CMakeLists.txt

@@ -80,6 +80,19 @@ add_entrypoint_object(
     libc.src.errno.errno
 )
 
+add_entrypoint_object(
+  faccessat
+  SRCS
+    faccessat.cpp
+  HDRS
+    ../faccessat.h
+  DEPENDS
+    libc.hdr.fcntl_macros
+    libc.include.sys_syscall
+    libc.src.__support.OSUtil.osutil
+    libc.src.errno.errno
+)
+
 add_entrypoint_object(
   fchdir
   SRCS

libc/src/unistd/linux/access.cpp

@@ -23,7 +23,7 @@ LLVM_LIBC_FUNCTION(int, access, (const char *path, int mode)) {
   int ret = LIBC_NAMESPACE::syscall_impl<int>(SYS_access, path, mode);
 #elif defined(SYS_faccessat)
   int ret =
-      LIBC_NAMESPACE::syscall_impl<int>(SYS_faccessat, AT_FDCWD, path, mode, 0);
+      LIBC_NAMESPACE::syscall_impl<int>(SYS_faccessat, AT_FDCWD, path, mode);
 #else
 #error "access and faccessat syscalls not available."
 #endif

libc/src/unistd/linux/faccessat.cpp

@@ -0,0 +1,37 @@
+//===-- Linux implementation of faccessat ---------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/unistd/faccessat.h"
+
+#include "src/__support/OSUtil/syscall.h" // For internal syscall function.
+#include "src/__support/common.h"
+
+#include "hdr/fcntl_macros.h"
+#include "src/__support/libc_errno.h"
+#include "src/__support/macros/config.h"
+#include <sys/syscall.h> // For syscall numbers.
+
+namespace LIBC_NAMESPACE_DECL {
+
+LLVM_LIBC_FUNCTION(int, faccessat,
+                   (int fd, const char *path, int amode, int flag)) {
+#ifdef SYS_faccessat2
+  int ret =
+      LIBC_NAMESPACE::syscall_impl<int>(SYS_faccessat2, fd, path, amode, flag);
+#else
+#error "faccessat2 syscall is not available."
+#endif
+
+  if (ret < 0) {
+    libc_errno = -ret;
+    return -1;
+  }
+  return 0;
+}
+
+} // namespace LIBC_NAMESPACE_DECL