Skip procfs symlinks in virtual proc_path resolution

src/runtime/procemu.c

@@ -3436,38 +3436,6 @@ int proc_intercept_open(const guest_t *g,
     return PROC_NOT_INTERCEPTED;
 }
 
-int proc_intercept_statfs(const char *path, struct statfs *out)
-{
-    /* /proc/<our_pid>[/...] -> /proc/self[...], same alias as
-     * proc_intercept_open/proc_intercept_stat.
-     */
-    char alias[LINUX_PATH_MAX];
-    int aliased = proc_alias_self(path, alias, sizeof(alias));
-    if (aliased < 0)
-        return -1;
-    if (aliased > 0)
-        return proc_intercept_statfs(alias, out);
-
-    /* /proc/self/fd and /proc/self/fdinfo are the only /proc nodes whose open
-     * path allocates per-call scratch state: proc_open_fd_scratch mkdtemp's a
-     * fresh directory and creates one placeholder file per live guest fd on
-     * every single open, purely so getdents has something to enumerate. A
-     * plain statfs() never enumerates the directory, so paying for that
-     * allocation just to fstatfs() the result and immediately discard it is
-     * wasted work -- and once PROC_SCRATCH_DIRS_MAX untracked opens
-     * accumulate, the atexit cleanup no longer knows about further dirs and
-     * they leak in /tmp permanently.
-     *
-     * Every scratch dir (like every other synthetic /proc node) is created
-     * under /tmp, so statfs("/tmp") reports identical filesystem info without
-     * allocating anything.
-     */
-    if (strcmp(path, "/proc/self/fd") && strcmp(path, "/proc/self/fd/") &&
-        strcmp(path, "/proc/self/fdinfo") && strcmp(path, "/proc/self/fdinfo/"))
-        return PROC_NOT_INTERCEPTED;
-
-    return statfs("/tmp", out) < 0 ? -1 : 0;
-}
 
 int proc_intercept_stat(const char *path, struct stat *st)
 {

src/runtime/procemu.h

@@ -45,16 +45,6 @@ int proc_intercept_readlink(const char *path, char *buf, size_t bufsiz);
  */
 int proc_intercept_stat(const char *path, struct stat *mac_st);
 
-/* Intercept statfs for /proc paths that would otherwise route through
- * proc_intercept_open purely to obtain filesystem-level info, forcing
- * allocation of scratch state (e.g. /proc/self/fd's per-open directory of
- * placeholder files, one per live guest fd) that statfs() has no use for.
- * Returns 0 if the filesystem info was synthesized (mac_st filled), or -2
- * (PROC_NOT_INTERCEPTED) if the path is not intercepted (fall through to the
- * normal open/fstatfs/close path).
- */
-int proc_intercept_statfs(const char *path, struct statfs *mac_st);
-
 /* Intercept writes to synthetic proc files that need stateful behavior.
  * Returns 1 if handled (with *written_out set), 0 if not intercepted, or -1 on
  * error with errno set.

src/syscall/fs-stat.c

@@ -381,11 +381,13 @@ static void fill_proc_statfs(linux_statfs_t *lin)
     lin->f_frsize = 4096;
 }
 
-int64_t sys_statfs(guest_t *g, uint64_t path_gva, uint64_t buf_gva)
+static int64_t sys_statfs_impl(guest_t *g,
+                               const char *path,
+                               uint64_t buf_gva,
+                               int depth)
 {
-    char path[LINUX_PATH_MAX];
-    if (guest_read_str_small(g, path_gva, path, sizeof(path)) < 0)
-        return -LINUX_EFAULT;
+    if (depth > 40)
+        return -LINUX_ELOOP;
 
     path_translation_t tx;
     if (path_translate_at(LINUX_AT_FDCWD, path, PATH_TR_NONE, &tx) < 0)
@@ -394,11 +396,37 @@ int64_t sys_statfs(guest_t *g, uint64_t path_gva, uint64_t buf_gva)
         return -LINUX_ENOSYS;
 
     if (statfs_path_is_proc(tx.intercept_path)) {
-        linux_statfs_t lin_st;
-        fill_proc_statfs(&lin_st);
-        if (guest_write_small(g, buf_gva, &lin_st, sizeof(lin_st)) < 0)
-            return -LINUX_EFAULT;
-        return 0;
+        if (proc_path_is_symlink(tx.intercept_path)) {
+            char link[LINUX_PATH_MAX];
+            int len = proc_intercept_readlink(tx.intercept_path, link,
+                                              sizeof(link) - 1);
+            if (len < 0)
+                return linux_errno();
+            link[len] = '\0';
+            return sys_statfs_impl(g, link, buf_gva, depth + 1);
+        }
+
+        struct stat mac_st;
+        int intercepted = proc_intercept_stat(tx.intercept_path, &mac_st);
+        if (intercepted == 0) {
+            linux_statfs_t lin_st;
+            fill_proc_statfs(&lin_st);
+            if (guest_write_small(g, buf_gva, &lin_st, sizeof(lin_st)) < 0)
+                return -LINUX_EFAULT;
+            return 0;
+        }
+        if (intercepted == -1)
+            return linux_errno();
+
+        /* It might be /proc itself or a host-backed file/directory under /proc
+         */
+        if (stat(tx.host_path, &mac_st) == 0) {
+            linux_statfs_t lin_st;
+            fill_proc_statfs(&lin_st);
+            if (guest_write_small(g, buf_gva, &lin_st, sizeof(lin_st)) < 0)
+                return -LINUX_EFAULT;
+            return 0;
+        }
     }
 
     struct statfs mac_st;
@@ -413,6 +441,15 @@ int64_t sys_statfs(guest_t *g, uint64_t path_gva, uint64_t buf_gva)
     return 0;
 }
 
+int64_t sys_statfs(guest_t *g, uint64_t path_gva, uint64_t buf_gva)
+{
+    char path[LINUX_PATH_MAX];
+    if (guest_read_str_small(g, path_gva, path, sizeof(path)) < 0)
+        return -LINUX_EFAULT;
+
+    return sys_statfs_impl(g, path, buf_gva, 0);
+}
+
 int64_t sys_fstatfs(guest_t *g, int fd, uint64_t buf_gva)
 {
     fd_entry_t snap;

src/syscall/fs.c

@@ -127,7 +127,7 @@ static const char *proc_stateful_file_path(const char *path)
     return NULL;
 }
 
-static bool proc_path_is_symlink(const char *path)
+bool proc_path_is_symlink(const char *path)
 {
     if (!path)
         return false;

src/syscall/internal.h

@@ -461,8 +461,8 @@ int64_t host_iov_prepare_msg(guest_t *g,
                              int required_perms,
                              host_iov_buf_t *buf);
 
-/* Release any heap spillover backing a host_iov_buf_t. Idempotent. */
 void host_iov_free(host_iov_buf_t *buf);
+bool proc_path_is_symlink(const char *path);
 
 /* Read a guest path string with small-buffer optimization.
  *

src/syscall/path.c

@@ -879,6 +879,10 @@ static int dirfd_guest_base_path(guest_fd_t dirfd, char *out, size_t outsz)
         return -1;
     }
     if (snap.proc_path[0] != '\0') {
+        if (snap.type != FD_DIR) {
+            errno = ENOTDIR;
+            return -1;
+        }
         size_t len = str_copy_trunc(out, snap.proc_path, outsz);
         if (len >= outsz) {
             errno = ENAMETOOLONG;

tests/test-proc.c

@@ -208,28 +208,35 @@ int main(void)
     TEST("statfs /proc");
     {
         struct statfs st;
-        if (statfs("/proc", &st) < 0)
+        if (statfs("/proc", &st) < 0) {
             FAIL("statfs failed");
-        else
-            PASS();
+        } else {
+            EXPECT_TRUE(st.f_type == 0x9fa0,
+                        "statfs /proc f_type not PROC_SUPER_MAGIC");
+        }
     }
 
     TEST("statfs /proc/ (trailing slash)");
     {
         struct statfs st;
-        if (statfs("/proc/", &st) < 0)
+        if (statfs("/proc/", &st) < 0) {
             FAIL("statfs failed");
-        else
-            PASS();
+        } else {
+            EXPECT_TRUE(st.f_type == 0x9fa0,
+                        "statfs /proc/ f_type not PROC_SUPER_MAGIC");
+        }
     }
 
     TEST("statfs /proc/self/cmdline");
     {
         struct statfs st;
-        if (statfs("/proc/self/cmdline", &st) < 0)
+        if (statfs("/proc/self/cmdline", &st) < 0) {
             FAIL("statfs failed");
-        else
-            PASS();
+        } else {
+            EXPECT_TRUE(
+                st.f_type == 0x9fa0,
+                "statfs /proc/self/cmdline f_type not PROC_SUPER_MAGIC");
+        }
     }
 
     TEST("statfs matches fstatfs for /proc/self/cmdline");
@@ -245,6 +252,36 @@ int main(void)
         } else {
             EXPECT_TRUE(path_st.f_type == fd_st.f_type,
                         "statfs/fstatfs f_type mismatch");
+            EXPECT_TRUE(path_st.f_type == 0x9fa0,
+                        "statfs f_type not PROC_SUPER_MAGIC");
+        }
+        if (fd >= 0)
+            close(fd);
+    }
+
+    TEST("statfs follows /proc symlinks");
+    {
+        struct statfs exe_st, fd_st;
+        int fd = open("/proc/self/exe", O_RDONLY);
+        if (fd < 0) {
+            FAIL("open /proc/self/exe failed");
+        } else if (statfs("/proc/self/exe", &exe_st) < 0) {
+            FAIL("statfs /proc/self/exe failed");
+        } else {
+            /* /proc/self/exe points to the host binary file, which resides on a
+             * real filesystem, not procfs. */
+            EXPECT_TRUE(exe_st.f_type != 0x9fa0,
+                        "statfs /proc/self/exe returned PROC_SUPER_MAGIC");
+
+            /* Build /proc/self/fd/N path */
+            char fd_path[64];
+            snprintf(fd_path, sizeof(fd_path), "/proc/self/fd/%d", fd);
+            if (statfs(fd_path, &fd_st) < 0) {
+                FAIL("statfs /proc/self/fd/N failed");
+            } else {
+                EXPECT_TRUE(fd_st.f_type != 0x9fa0,
+                            "statfs /proc/self/fd/N returned PROC_SUPER_MAGIC");
+            }
         }
         if (fd >= 0)
             close(fd);