Kernel+LibC+LibCore+UE: Implement fchmodat(2)

This function is an extended version of `chmod(2)` that lets one control whether to dereference symlinks, and specify a file descriptor to a directory that will be used as the base for relative paths.
akimdi · Jan 12, 2022 · 182016d · 182016d
1 parent 5f71925
commit 182016d
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Showing 10 changed files with 64 additions and 14 deletions.
diff --git a/Kernel/API/Syscall.h b/Kernel/API/Syscall.h
@@ -478,6 +478,13 @@ struct SC_statvfs_params {
  struct statvfs* buf;
 };
 
+struct SC_chmod_params {
+ int dirfd;
+ StringArgument path;
+ u16 mode;
+ int follow_symlinks;
+};
+
 void initialize();
 int sync();
 

diff --git a/Kernel/FileSystem/VirtualFileSystem.cpp b/Kernel/FileSystem/VirtualFileSystem.cpp
@@ -434,9 +434,9 @@ ErrorOr<void> VirtualFileSystem::chmod(Custody& custody, mode_t mode)
  return inode.chmod(mode);
 }
 
-ErrorOr<void> VirtualFileSystem::chmod(StringView path, mode_t mode, Custody& base)
+ErrorOr<void> VirtualFileSystem::chmod(StringView path, mode_t mode, Custody& base, int options)
 {
- auto custody = TRY(resolve_path(path, base));
+ auto custody = TRY(resolve_path(path, base, nullptr, options));
  return chmod(custody, mode);
 }
 

diff --git a/Kernel/FileSystem/VirtualFileSystem.h b/Kernel/FileSystem/VirtualFileSystem.h
@@ -57,7 +57,7 @@ class VirtualFileSystem {
  ErrorOr<void> unlink(StringView path, Custody& base);
  ErrorOr<void> symlink(StringView target, StringView linkpath, Custody& base);
  ErrorOr<void> rmdir(StringView path, Custody& base);
- ErrorOr<void> chmod(StringView path, mode_t, Custody& base);
+ ErrorOr<void> chmod(StringView path, mode_t, Custody& base, int options = 0);
  ErrorOr<void> chmod(Custody&, mode_t);
  ErrorOr<void> chown(StringView path, UserID, GroupID, Custody& base, int options);
  ErrorOr<void> chown(Custody&, UserID, GroupID);

diff --git a/Kernel/Process.h b/Kernel/Process.h
@@ -359,7 +359,7 @@ class Process final
  ErrorOr<FlatPtr> sys$rmdir(Userspace<const char*> pathname, size_t path_length);
  ErrorOr<FlatPtr> sys$mount(Userspace<const Syscall::SC_mount_params*>);
  ErrorOr<FlatPtr> sys$umount(Userspace<const char*> mountpoint, size_t mountpoint_length);
- ErrorOr<FlatPtr> sys$chmod(Userspace<const char*> pathname, size_t path_length, mode_t);
+ ErrorOr<FlatPtr> sys$chmod(Userspace<Syscall::SC_chmod_params const*>);
  ErrorOr<FlatPtr> sys$fchmod(int fd, mode_t);
  ErrorOr<FlatPtr> sys$chown(Userspace<const Syscall::SC_chown_params*>);
  ErrorOr<FlatPtr> sys$fchown(int fd, UserID, GroupID);

diff --git a/Kernel/Syscalls/chmod.cpp b/Kernel/Syscalls/chmod.cpp
@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2018-2020, Andreas Kling <[email protected]>
+ * Copyright (c) 2022, Daniel Bertalan <[email protected]>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -10,12 +11,24 @@
 
 namespace Kernel {
 
-ErrorOr<FlatPtr> Process::sys$chmod(Userspace<const char*> user_path, size_t path_length, mode_t mode)
+ErrorOr<FlatPtr> Process::sys$chmod(Userspace<Syscall::SC_chmod_params const*> user_params)
 {
  VERIFY_PROCESS_BIG_LOCK_ACQUIRED(this);
  TRY(require_promise(Pledge::fattr));
- auto path = TRY(get_syscall_path_argument(user_path, path_length));
- TRY(VirtualFileSystem::the().chmod(path->view(), mode, current_directory()));
+ auto params = TRY(copy_typed_from_user(user_params));
+ auto path = TRY(get_syscall_path_argument(params.path));
+
+ RefPtr<Custody> base;
+ if (params.dirfd == AT_FDCWD) {
+ base = current_directory();
+ } else {
+ auto base_description = TRY(fds().open_file_description(params.dirfd));
+ if (!base_description->custody())
+ return EINVAL;
+ base = base_description->custody();
+ }
+
+ TRY(VirtualFileSystem::the().chmod(path->view(), params.mode, *base, params.follow_symlinks ? 0 : O__NOERROR));
  return 0;
 }
 

diff --git a/Userland/DevTools/UserspaceEmulator/Emulator.h b/Userland/DevTools/UserspaceEmulator/Emulator.h
@@ -136,7 +136,7 @@ class Emulator {
  int virt$beep();
  int virt$bind(int sockfd, FlatPtr address, socklen_t address_length);
  int virt$chdir(FlatPtr, size_t);
- int virt$chmod(FlatPtr, size_t, mode_t);
+ int virt$chmod(FlatPtr);
  int virt$chown(FlatPtr);
  int virt$clock_gettime(int, FlatPtr);
  int virt$clock_nanosleep(FlatPtr);

diff --git a/Userland/DevTools/UserspaceEmulator/Emulator_syscalls.cpp b/Userland/DevTools/UserspaceEmulator/Emulator_syscalls.cpp
@@ -52,7 +52,7 @@ u32 Emulator::virt_syscall(u32 function, u32 arg1, u32 arg2, u32 arg3)
  case SC_chdir:
  return virt$chdir(arg1, arg2);
  case SC_chmod:
- return virt$chmod(arg1, arg2, arg3);
+ return virt$chmod(arg1);
  case SC_chown:
  return virt$chown(arg1);
  case SC_clock_gettime:
@@ -418,10 +418,15 @@ int Emulator::virt$dbgputstr(FlatPtr characters, int length)
  return 0;
 }
 
-int Emulator::virt$chmod(FlatPtr path_addr, size_t path_length, mode_t mode)
+int Emulator::virt$chmod(FlatPtr params_addr)
 {
- auto path = mmu().copy_buffer_from_vm(path_addr, path_length);
- return syscall(SC_chmod, path.data(), path.size(), mode);
+ Syscall::SC_chmod_params params;
+ mmu().copy_from_vm(&params, params_addr, sizeof(params));
+
+ auto path = mmu().copy_buffer_from_vm((FlatPtr)params.path.characters, params.path.length);
+ params.path.characters = (char const*)path.data();
+ params.path.length = path.size();
+ return syscall(SC_chmod, &params);
 }
 
 int Emulator::virt$chown(FlatPtr params_addr)

diff --git a/Userland/Libraries/LibC/stat.cpp b/Userland/Libraries/LibC/stat.cpp
@@ -34,12 +34,30 @@ int mkdir(const char* pathname, mode_t mode)
 
 // https://pubs.opengroup.org/onlinepubs/9699919799/functions/chmod.html
 int chmod(const char* pathname, mode_t mode)
+{
+ return fchmodat(AT_FDCWD, pathname, mode, 0);
+}
+
+// https://pubs.opengroup.org/onlinepubs/9699919799/functions/fchmodat.html
+int fchmodat(int dirfd, char const* pathname, mode_t mode, int flags)
 {
  if (!pathname) {
  errno = EFAULT;
  return -1;
  }
- int rc = syscall(SC_chmod, pathname, strlen(pathname), mode);
+
+ if (flags & ~AT_SYMLINK_) {
+ errno = EINVAL;
+ return -1;
+ }
+
+ Syscall::SC_chmod_params params {
+ dirfd,
+ { pathname, strlen(pathname) },
+ mode,
+ !(flags & AT_SYMLINK_)
+ };
+ int rc = syscall(SC_chmod, &params);
  __RETURN_WITH_ERRNO(rc, rc, -1);
 }
 

diff --git a/Userland/Libraries/LibC/sys/stat.h b/Userland/Libraries/LibC/sys/stat.h
@@ -15,6 +15,7 @@ __BEGIN_DECLS
 
 mode_t umask(mode_t);
 int chmod(const char* pathname, mode_t);
+int fchmodat(int fd, char const* path, mode_t mode, int flag);
 int fchmod(int fd, mode_t);
 int mkdir(const char* pathname, mode_t);
 int mkfifo(const char* pathname, mode_t);

diff --git a/Userland/Libraries/LibCore/System.cpp b/Userland/Libraries/LibCore/System.cpp
@@ -386,7 +386,13 @@ ErrorOr<void> chmod(StringView pathname, mode_t mode)
  return Error::from_syscall("chmod"sv, -EFAULT);
 
 #ifdef __serenity__
- int rc = syscall(SC_chmod, pathname.characters_without_null_termination(), pathname.length(), mode);
+ Syscall::SC_chmod_params params {
+ AT_FDCWD,
+ { pathname.characters_without_null_termination(), pathname.length() },
+ mode,
+ true
+ };
+ int rc = syscall(SC_chmod, &params);
  HANDLE_SYSCALL_RETURN_VALUE("chmod"sv, rc, {});
 #else
  String path = pathname;