kernel+libc+libos+ls: Add readlink()

apps/ls.cpp

@@ -85,6 +85,8 @@ Result<int> luna_main(int argc, char** argv)
             struct stat st;
             TRY(os::FileSystem::stat({ dirfd, file.view() }, st, false));
 
+            auto link = TRY(os::FileSystem::readlink({ dirfd, file.view() }));
+
             StringView owner;
             StringView group;
 
@@ -92,14 +94,14 @@ Result<int> luna_main(int argc, char** argv)
 
             if (!human_readable && !si)
             {
-                os::println("%6o %u %4s %4s %10lu %s", st.st_mode, st.st_nlink, owner.chars(), group.chars(),
-                            st.st_size, file.chars());
+                os::println("%6o %u %4s %4s %10lu %s%s%s", st.st_mode, st.st_nlink, owner.chars(), group.chars(),
+                            st.st_size, file.chars(), link.is_empty() ? "" : " -> ", link.chars());
             }
             else
             {
                 auto size = TRY(to_dynamic_unit(st.st_size, 10, false, si ? Unit::SI : Unit::Binary, false));
-                os::println("%6o %u %4s %4s %6s %s", st.st_mode, st.st_nlink, owner.chars(), group.chars(),
-                            size.chars(), file.chars());
+                os::println("%6o %u %4s %4s %6s %s%s%s", st.st_mode, st.st_nlink, owner.chars(), group.chars(),
+                            size.chars(), file.chars(), link.is_empty() ? "" : " -> ", link.chars());
             }
         }
     }

kernel/src/sys/link.cpp

@@ -62,3 +62,29 @@ Result<u64> sys_symlinkat(Registers*, SyscallArgs args)
 
     return 0;
 }
+
+Result<u64> sys_readlinkat(Registers*, SyscallArgs args)
+{
+    int dirfd = (int)args[0];
+    auto path = TRY(MemoryManager::strdup_from_user(args[1]));
+    char* buf = (char*)args[2];
+    usize bufsiz = (usize)args[3];
+
+    auto* current = Scheduler::current();
+
+    auto symlink = TRY(current->resolve_atfile(dirfd, path, true, false));
+
+    if (symlink->type() != VFS::InodeType::Symlink) return err(EINVAL);
+
+    auto linkpath = TRY(symlink->readlink());
+    check(!linkpath.is_empty());
+
+    usize nread = linkpath.length();
+    if (nread > bufsiz) nread = bufsiz;
+
+    kdbgln("readlink: reading %zu bytes from symlink (%s)", nread, linkpath.chars());
+
+    if (!MemoryManager::copy_to_user(buf, linkpath.chars(), nread)) return err(EFAULT);
+
+    return nread;
+}

libc/include/unistd.h

@@ -139,6 +139,12 @@ extern "C"
     /* Create a symbolic link relative to a file descriptor. */
     int symlinkat(const char* target, int dirfd, const char* linkpath);
 
+    /* Read the contents of a symbolic link. */
+    ssize_t readlink(const char* path, char* buf, size_t max);
+
+    /* Read the contents of a symbolic link relative to a file descriptor. */
+    ssize_t readlinkat(int dirfd, const char* path, char* buf, size_t max);
+
 #ifdef __cplusplus
 }
 #endif

libc/src/unistd.cpp

@@ -418,4 +418,16 @@ extern "C"
         long rc = syscall(SYS_symlinkat, target, dirfd, linkpath);
         __errno_return(rc, int);
     }
+
+    ssize_t readlink(const char* path, char* buf, size_t max)
+    {
+        long rc = syscall(SYS_readlinkat, AT_FDCWD, path, buf, max);
+        __errno_return(rc, ssize_t);
+    }
+
+    ssize_t readlinkat(int dirfd, const char* path, char* buf, size_t max)
+    {
+        long rc = syscall(SYS_readlinkat, dirfd, path, buf, max);
+        __errno_return(rc, ssize_t);
+    }
 }

libluna/include/luna/Syscall.h

@@ -5,7 +5,7 @@
         _e(lseek) _e(mkdir) _e(execve) _e(fork) _e(waitpid) _e(getppid) _e(fcntl) _e(getdents) _e(getuid) _e(geteuid)  \
             _e(getgid) _e(getegid) _e(setuid) _e(setgid) _e(seteuid) _e(setegid) _e(fchmodat) _e(fchownat) _e(ioctl)   \
                 _e(fstatat) _e(chdir) _e(getcwd) _e(unlinkat) _e(uname) _e(sethostname) _e(dup2) _e(pipe) _e(mount)    \
-                    _e(umount) _e(pstat) _e(getrusage) _e(symlinkat)
+                    _e(umount) _e(pstat) _e(getrusage) _e(symlinkat) _e(readlinkat)
 
 enum Syscalls
 {

libos/include/os/FileSystem.h

@@ -21,6 +21,8 @@ namespace os
 
         Result<void> remove_tree(const Path& path);
 
+        Result<String> readlink(const Path& path);
+
         Result<String> working_directory();
         Result<String> home_directory();
 

libos/src/FileSystem.cpp

@@ -2,6 +2,7 @@
 #include <dirent.h>
 #include <errno.h>
 #include <fcntl.h>
+#include <luna/PathParser.h>
 #include <luna/String.h>
 #include <os/Directory.h>
 #include <os/FileSystem.h>
@@ -68,6 +69,23 @@ namespace os::FileSystem
         return remove(path);
     }
 
+    Result<String> readlink(const Path& path)
+    {
+        struct stat st;
+        TRY(stat(path, st, false));
+        if (!S_ISLNK(st.st_mode)) return String {};
+
+        char* buf = TRY(make_array<char>(st.st_size + 1));
+        auto guard = make_scope_guard([buf] { delete[] buf; });
+        usize nread = TRY(
+            Result<usize>::from_syscall(syscall(SYS_readlinkat, path.dirfd(), path.name().chars(), buf, st.st_size)));
+
+        buf[nread] = '\0';
+        guard.deactivate();
+
+        return String { buf, nread };
+    }
+
     Result<String> working_directory()
     {
         char* ptr = getcwd(NULL, 0);