[LibOS] Add readlinkat syscall implementation

LibOS/shim/include/shim_table.h

@@ -410,7 +410,7 @@ int shim_do_mkdir(const char* pathname, int mode);
 int shim_do_rmdir(const char* pathname);
 int shim_do_creat(const char* path, mode_t mode);
 int shim_do_unlink(const char* file);
-int shim_do_readlink(const char* file, char* buf, size_t bufsize);
+int shim_do_readlink(const char* file, char* buf, int bufsize);
 int shim_do_chmod(const char* filename, mode_t mode);
 int shim_do_fchmod(int fd, mode_t mode);
 int shim_do_chown(const char* filename, uid_t user, gid_t group);
@@ -476,6 +476,7 @@ int shim_do_openat(int dfd, const char* filename, int flags, int mode);
 int shim_do_mkdirat(int dfd, const char* pathname, int mode);
 int shim_do_newfstatat(int dirfd, const char* pathname, struct stat* statbuf, int flags);
 int shim_do_unlinkat(int dfd, const char* pathname, int flag);
+int shim_do_readlinkat(int dirfd, const char* file, char* buf, int bufsize);
 int shim_do_renameat(int olddfd, const char* pathname, int newdfd, const char* newname);
 int shim_do_fchmodat(int dfd, const char* filename, mode_t mode);
 int shim_do_fchownat(int dfd, const char* filename, uid_t user, gid_t group, int flags);
@@ -599,7 +600,7 @@ int shim_creat(const char* path, mode_t mode);
 int shim_link(const char* oldname, const char* newname);
 int shim_unlink(const char* file);
 int shim_symlink(const char* old, const char* new);
-int shim_readlink(const char* file, char* buf, size_t bufsize);
+int shim_readlink(const char* file, char* buf, int bufsize);
 int shim_chmod(const char* filename, mode_t mode);
 int shim_fchmod(int fd, mode_t mode);
 int shim_chown(const char* filename, uid_t user, gid_t group);

LibOS/shim/src/shim_syscalls.c

@@ -392,7 +392,7 @@ DEFINE_SHIM_SYSCALL(unlink, 1, shim_do_unlink, int, const char*, file)
 SHIM_SYSCALL_RETURN_ENOSYS(symlink, 2, int, const char*, old, const char*, new)
 
 /* readlink: sys/shim_stat.c */
-DEFINE_SHIM_SYSCALL(readlink, 3, shim_do_readlink, int, const char*, path, char*, buf, size_t,
+DEFINE_SHIM_SYSCALL(readlink, 3, shim_do_readlink, int, const char*, path, char*, buf, int,
                     bufsize)
 
 DEFINE_SHIM_SYSCALL(chmod, 2, shim_do_chmod, int, const char*, filename, mode_t, mode)
@@ -904,7 +904,8 @@ SHIM_SYSCALL_RETURN_ENOSYS(linkat, 5, int, int, olddfd, const char*, oldname, in
 SHIM_SYSCALL_RETURN_ENOSYS(symlinkat, 3, int, const char*, oldname, int, newdfd, const char*,
                            newname)
 
-SHIM_SYSCALL_RETURN_ENOSYS(readlinkat, 4, int, int, dfd, const char*, path, char*, buf, int, bufsiz)
+DEFINE_SHIM_SYSCALL(readlinkat, 4, shim_do_readlinkat, int, int, dfd, const char*, path, char*, buf,
+                    int, bufsiz)
 
 /* fchmodat: sys/shim_fs.c */
 DEFINE_SHIM_SYSCALL(fchmodat, 3, shim_do_fchmodat, int, int, dfd, const char*, filename, mode_t,

LibOS/shim/src/sys/shim_stat.c

@@ -92,22 +92,27 @@ out:
     return ret;
 }
 
-int shim_do_readlink(const char* file, char* buf, size_t bufsize) {
+int shim_do_readlinkat(int dirfd, const char* file, char* buf, int bufsize) {
     if (!file || test_user_string(file))
         return -EFAULT;
 
-    if (!buf || !bufsize || test_user_memory(buf, bufsize, true))
-        return -EFAULT;
-
     if (bufsize <= 0)
         return -EINVAL;
 
-    int ret;
+    if (test_user_memory(buf, bufsize, true))
+        return -EFAULT;
+
     struct shim_dentry* dent = NULL;
+    struct shim_dentry* dir = NULL;
+    int ret = get_dirfd_dentry(dirfd, &dir);
+    if (ret < 0) {
+        goto out;
+    }
+
     struct shim_qstr qstr = QSTR_INIT;
 
-    if ((ret = path_lookupat(NULL, file, LOOKUP_ACCESS, &dent, NULL)) < 0)
-        return ret;
+    if ((ret = path_lookupat(dir, file, LOOKUP_ACCESS, &dent, NULL)) < 0)
+        goto out;
 
     ret = -EINVAL;
     /* The correct behavior is to return -EINVAL if file is not a
@@ -122,17 +127,25 @@ int shim_do_readlink(const char* file, char* buf, size_t bufsize) {
     if (ret < 0)
         goto out;
 
-    ret = -ENAMETOOLONG;
-    if (qstr.len >= bufsize)
-        goto out;
+    ret = bufsize;
+    if (qstr.len < (size_t)bufsize)
+        ret = qstr.len;
 
-    memcpy(buf, qstrgetstr(&qstr), qstr.len);
-    ret = qstr.len;
+    memcpy(buf, qstrgetstr(&qstr), ret);
 out:
-    put_dentry(dent);
+    if (dent) {
+        put_dentry(dent);
+    }
+    if (dir) {
+        put_dentry(dir);
+    }
     return ret;
 }
 
+int shim_do_readlink(const char* file, char* buf, int bufsize) {
+    return shim_do_readlinkat(AT_FDCWD, file, buf, bufsize);
+}
+
 static int __do_statfs(struct shim_mount* fs, struct statfs* buf) {
     __UNUSED(fs);
     if (!buf || test_user_memory(buf, sizeof(*buf), true))

LibOS/shim/test/regression/proc_common.c

@@ -1,4 +1,4 @@
-#define _XOPEN_SOURCE 700
+#define _GNU_SOURCE
 #include <dirent.h>
 #include <errno.h>
 #include <fcntl.h>
@@ -133,12 +133,23 @@ int main(int argc, char** argv) {
     }
 
     printf("===== Reading /proc/self/exe symlink\n");
-    ssize_t len = readlink("/proc/self/exe", buf, sizeof(buf) - 1);
+    int proc_dirfd = open("/proc", O_DIRECTORY | O_PATH | O_RDONLY);
+    if (proc_dirfd < 0) {
+        perror("open /proc");
+        return 1;
+    }
+
+    ssize_t len = readlinkat(proc_dirfd, "self/exe", buf, sizeof(buf) - 1);
     if (len < 0) {
         perror("readlink /proc/self/exe");
         return 1;
     }
 
+    if (close(proc_dirfd) < 0) {
+        perror("close proc_dirfd");
+        return 1;
+    }
+
     buf[len] = '\0';
     printf("symlink /proc/self/exec resolves to %s\n", buf);