Kernel+LibC: Implement `readlinkat(2)`

Co-Authored-By: Daniel Bertalan <dani@danielbertalan.dev>

Kernel/API/Syscall.h

@@ -389,6 +389,7 @@ struct SC_execve_params {
 struct SC_readlink_params {
     StringArgument path;
     MutableBufferArgument<char, size_t> buffer;
+    int dirfd;
 };
 
 struct SC_link_params {

Kernel/Syscalls/readlink.cpp

@@ -17,7 +17,7 @@ ErrorOr<FlatPtr> Process::sys$readlink(Userspace<Syscall::SC_readlink_params con
     auto params = TRY(copy_typed_from_user(user_params));
 
     auto path = TRY(get_syscall_path_argument(params.path));
-    auto description = TRY(VirtualFileSystem::the().open(credentials(), path->view(), O_RDONLY | O_NOFOLLOW_NOERROR, 0, current_directory()));
+    auto description = TRY(VirtualFileSystem::the().open(credentials(), path->view(), O_RDONLY | O_NOFOLLOW_NOERROR, 0, TRY(custody_for_dirfd(params.dirfd))));
 
     if (!description->metadata().is_symlink())
         return EINVAL;

Userland/Libraries/LibC/serenity.cpp

@@ -105,7 +105,8 @@ int serenity_readlink(char const* path, size_t path_length, char* buffer, size_t
 {
     Syscall::SC_readlink_params small_params {
         { path, path_length },
-        { buffer, buffer_size }
+        { buffer, buffer_size },
+        AT_FDCWD
     };
     int rc = syscall(SC_readlink, &small_params);
     __RETURN_WITH_ERRNO(rc, rc, -1);

Userland/Libraries/LibC/unistd.cpp

@@ -630,10 +630,16 @@ int sethostname(char const* hostname, ssize_t size)
     __RETURN_WITH_ERRNO(rc, rc, -1);
 }
 
-// https://pubs.opengroup.org/onlinepubs/9699919799/functions/readlink.html
+// https://pubs.opengroup.org/onlinepubs/9699919799/functions/readlinkat.html
 ssize_t readlink(char const* path, char* buffer, size_t size)
 {
-    Syscall::SC_readlink_params params { { path, strlen(path) }, { buffer, size } };
+    return readlinkat(AT_FDCWD, path, buffer, size);
+}
+
+// https://pubs.opengroup.org/onlinepubs/9699919799/functions/readlinkat.html
+ssize_t readlinkat(int dirfd, char const* path, char* buffer, size_t size)
+{
+    Syscall::SC_readlink_params params { { path, strlen(path) }, { buffer, size }, dirfd };
     int rc = syscall(SC_readlink, &params);
     // Return the number of bytes placed in the buffer, not the full path size.
     __RETURN_WITH_ERRNO(rc, min((size_t)rc, size), -1);

Userland/Libraries/LibC/unistd.h

@@ -88,6 +88,7 @@ int usleep(useconds_t);
 int gethostname(char*, size_t);
 int sethostname(char const*, ssize_t);
 ssize_t readlink(char const* path, char* buffer, size_t);
+ssize_t readlinkat(int dirfd, char const* path, char* buffer, size_t);
 char* ttyname(int fd);
 int ttyname_r(int fd, char* buffer, size_t);
 off_t lseek(int fd, off_t, int whence);

Userland/Libraries/LibCore/System.cpp

@@ -1386,8 +1386,9 @@ ErrorOr<DeprecatedString> readlink(StringView pathname)
     char data[PATH_MAX];
 #ifdef AK_OS_SERENITY
     Syscall::SC_readlink_params small_params {
-        { pathname.characters_without_null_termination(), pathname.length() },
-        { data, sizeof(data) }
+        .path = { pathname.characters_without_null_termination(), pathname.length() },
+        .buffer = { data, sizeof(data) },
+        .dirfd = AT_FDCWD,
     };
     int rc = syscall(SC_readlink, &small_params);
     HANDLE_SYSCALL_RETURN_VALUE("readlink", rc, DeprecatedString(data, rc));