linux-user: add openat2 support in linux-user

This commit adds support for the `openat2()` syscall in the
`linux-user` userspace emulator.

It is implemented by extracting a new helper `maybe_do_fake_open()`
out of the exiting `do_guest_openat()` and share that with the
new `do_guest_openat2()`. Unfortunatly we cannot just make
do_guest_openat2() a superset of do_guest_openat() because the
openat2() syscall is stricter with the argument checking and
will return an error for invalid flags or mode combinations (which
open()/openat() will ignore).

The implementation is similar to SYSCALL_DEFINE(openat2), i.e.
a new `copy_struct_from_user()` is usef that works the same
as the kernels version to support backwards-compatibility
for struct syscall argument.

Instead of including openat2.h we create a copy of `open_how`
as `open_how_ver0` to ensure that if the structure grows we
can log a LOG_UNIMP warning.

Note that in this commit using openat2() for a "faked" file in
/proc will ignore the "resolve" flags. This is not great but it
seems similar to the exiting behavior when openat() is called
with a dirfd to "/proc". Here too the fake file lookup may
not catch the special file because "realpath()" is used to
determine if the path is in /proc. Alternatively to ignoring
we could simply fail with `-TARGET_ENOSYS` (or similar) if
`resolve` flags are passed and we found something that looks
like a file in /proc that needs faking.

Signed-off-by: Michael Vogt <[email protected]>
Buglink: osbuild/bootc-image-builder#619

linux-user/syscall.c

@@ -602,6 +602,34 @@ static int check_zeroed_user(abi_long addr, size_t ksize, size_t usize)
     return 1;
 }
 
+/*
+ * Copies a target struct to a host struct, in a way that guarantees
+ * backwards-compatibility for struct syscall arguments.
+ *
+ * Similar to kernels uaccess.h:copy_struct_from_user()
+ */
+static int
+copy_struct_from_user(void *dst, size_t ksize, abi_ptr src, size_t usize)
+{
+    size_t size = MIN(ksize, usize);
+    size_t rest = MAX(ksize, usize) - size;
+
+    /* Deal with trailing bytes. */
+    if (usize < ksize) {
+        memset(dst + size, 0, rest);
+    } else if (usize > ksize) {
+        int ret = check_zeroed_user(src, ksize, usize);
+        if (ret <= 0) {
+            return ret ?: -TARGET_E2BIG;
+        }
+    }
+    /* Copy the interoperable parts of the struct. */
+    if (copy_from_user(dst, src, size)) {
+        return -TARGET_EFAULT;
+    }
+    return 0;
+}
+
 #define safe_syscall0(type, name) \
 static type safe_##name(void) \
 { \
@@ -653,6 +681,15 @@ safe_syscall3(ssize_t, read, int, fd, void *, buff, size_t, count)
 safe_syscall3(ssize_t, write, int, fd, const void *, buff, size_t, count)
 safe_syscall4(int, openat, int, dirfd, const char *, pathname, \
               int, flags, mode_t, mode)
+
+struct open_how_ver0 {
+    __u64 flags;
+    __u64 mode;
+    __u64 resolve;
+};
+safe_syscall4(int, openat2, int, dirfd, const char *, pathname, \
+              const struct open_how_ver0 *, how, size_t, size)
+
 #if defined(TARGET_NR_wait4) || defined(TARGET_NR_waitpid)
 safe_syscall4(pid_t, wait4, pid_t, pid, int *, status, int, options, \
               struct rusage *, rusage)
@@ -8334,8 +8371,9 @@ static int open_net_route(CPUArchState *cpu_env, int fd)
 }
 #endif
 
-int do_guest_openat(CPUArchState *cpu_env, int dirfd, const char *fname,
-                    int flags, mode_t mode, bool safe)
+static int maybe_do_fake_open(CPUArchState *cpu_env, int dirfd,
+                              const char *fname, int flags, mode_t mode,
+                              bool safe, bool *use_returned_fd)
 {
     g_autofree char *proc_name = NULL;
     const char *pathname;
@@ -8362,6 +8400,7 @@ int do_guest_openat(CPUArchState *cpu_env, int dirfd, const char *fname,
 #endif
         { NULL, NULL, NULL }
     };
+    *use_returned_fd = true;
 
     /* if this is a file from /proc/ filesystem, expand full name */
     proc_name = realpath(fname, NULL);
@@ -8418,13 +8457,77 @@ int do_guest_openat(CPUArchState *cpu_env, int dirfd, const char *fname,
         return fd;
     }
 
+    *use_returned_fd = false;
+    return -1;
+}
+
+int do_guest_openat(CPUArchState *cpu_env, int dirfd, const char *fname,
+                    int flags, mode_t mode, bool safe)
+{
+    bool use_returned_fd;
+    int fd = maybe_do_fake_open(cpu_env, dirfd, fname, flags, mode, safe,
+                                &use_returned_fd);
+    if (use_returned_fd) {
+        return fd;
+    }
+
     if (safe) {
-        return safe_openat(dirfd, path(pathname), flags, mode);
+        return safe_openat(dirfd, path(fname), flags, mode);
     } else {
-        return openat(dirfd, path(pathname), flags, mode);
+        return openat(dirfd, path(fname), flags, mode);
     }
 }
 
+
+static int do_openat2(CPUArchState *cpu_env, abi_long dirfd,
+                      abi_ptr guest_pathname, abi_ptr guest_open_how,
+                      abi_long guest_size)
+{
+    struct open_how_ver0 how = {0};
+    int ret;
+
+    if (guest_size < sizeof(struct target_open_how_ver0)) {
+        return -TARGET_EINVAL;
+    }
+    ret = copy_struct_from_user(&how, sizeof(how), guest_open_how, guest_size);
+    if (ret) {
+        if (ret == -TARGET_E2BIG) {
+            qemu_log_mask(LOG_UNIMP,
+                          "Unimplemented openat2 open_how size: %lu\n",
+                          guest_size);
+        }
+        return ret;
+    }
+    char *pathname = lock_user_string(guest_pathname);
+    if (!pathname) {
+        return -TARGET_EFAULT;
+    }
+
+    how.flags = target_to_host_bitmask(how.flags, fcntl_flags_tbl);
+    how.mode = tswap64(how.mode);
+    how.resolve = tswap64(how.resolve);
+
+    /*
+     * Ideally we would pass "how->resolve" flags into this helper too but
+     * the lookup for files that need faking is based on "realpath()" so
+     * neither a dirfd for "proc" nor restrictions via "resolve" flags can
+     * be honored right now.
+     */
+    bool use_returned_fd;
+    int fd = maybe_do_fake_open(cpu_env, dirfd, pathname, how.flags, how.mode,
+                                true, &use_returned_fd);
+    if (use_returned_fd) {
+        return fd;
+    } else {
+        ret = get_errno(safe_openat2(dirfd, pathname, &how,
+                                     sizeof(struct open_how_ver0)));
+    }
+
+    fd_trans_unregister(ret);
+    unlock_user(pathname, guest_pathname, 0);
+    return ret;
+}
+
 ssize_t do_guest_readlink(const char *pathname, char *buf, size_t bufsiz)
 {
     ssize_t ret;
@@ -9197,6 +9300,11 @@ static abi_long do_syscall1(CPUArchState *cpu_env, int num, abi_long arg1,
         fd_trans_unregister(ret);
         unlock_user(p, arg2, 0);
         return ret;
+#if defined(TARGET_NR_openat2)
+    case TARGET_NR_openat2:
+        ret = do_openat2(cpu_env, arg1, arg2, arg3, arg4);
+        return ret;
+#endif
 #if defined(TARGET_NR_name_to_handle_at) && defined(CONFIG_OPEN_BY_HANDLE)
     case TARGET_NR_name_to_handle_at:
         ret = do_name_to_handle_at(arg1, arg2, arg3, arg4, arg5);

linux-user/syscall_defs.h

@@ -2754,4 +2754,11 @@ struct target_sched_param {
     abi_int sched_priority;
 };
 
+/* from kernel's include/uapi/linux/openat2.h */
+struct target_open_how_ver0 {
+    abi_ullong flags;
+    abi_ullong mode;
+    abi_ullong resolve;
+};
+
 #endif