Add Rust impl of wasmtime_ssp_fd_prestat_get

In more detail, this commit:
* makes fd_prestat_get safe
* rewrites fd_prestats_get_entry in (safe) Rust
* creates helper macros for rwlock read lock and unlock

wasmtime-wasi/build.rs

@@ -20,8 +20,10 @@ fn main() {
         .whitelist_function("fd_prestats_init")
         .whitelist_function("fd_prestats_insert")
         .whitelist_function("argv_environ_init")
+        .whitelist_function("rwlock_.*")
         .whitelist_type("__wasi_.*")
         .whitelist_type("fd_table")
+        .whitelist_type("fd_prestat")
         .whitelist_type("fd_prestats")
         .whitelist_type("argv_environ_values")
         .whitelist_var("__WASI_.*")

wasmtime-wasi/sandboxed-system-primitives/include/wasmtime_ssp.h

@@ -497,14 +497,6 @@ __wasi_errno_t wasmtime_ssp_environ_sizes_get(
     size_t *environ_buf_size
 ) WASMTIME_SSP_SYSCALL_NAME(environ_sizes_get) __attribute__((__warn_unused_result__));
 
-__wasi_errno_t wasmtime_ssp_fd_prestat_get(
-#if !defined(WASMTIME_SSP_STATIC_CURFDS)
-    struct fd_prestats *prestats,
-#endif
-    __wasi_fd_t fd,
-    __wasi_prestat_t *buf
-) WASMTIME_SSP_SYSCALL_NAME(fd_prestat_get) __attribute__((__warn_unused_result__));
-
 __wasi_errno_t wasmtime_ssp_fd_prestat_dir_name(
 #if !defined(WASMTIME_SSP_STATIC_CURFDS)
     struct fd_prestats *prestats,

wasmtime-wasi/sandboxed-system-primitives/src/locking.h

@@ -83,21 +83,21 @@ struct LOCKABLE rwlock {
   pthread_rwlock_t object;
 };
 
-static inline void rwlock_init(struct rwlock *lock) REQUIRES_UNLOCKED(*lock) {
+void rwlock_init(struct rwlock *lock) REQUIRES_UNLOCKED(*lock) {
   pthread_rwlock_init(&lock->object, NULL);
 }
 
-static inline void rwlock_rdlock(struct rwlock *lock)
+void rwlock_rdlock(struct rwlock *lock)
     LOCKS_SHARED(*lock) NO_LOCK_ANALYSIS {
   pthread_rwlock_rdlock(&lock->object);
 }
 
-static inline void rwlock_wrlock(struct rwlock *lock)
+void rwlock_wrlock(struct rwlock *lock)
     LOCKS_EXCLUSIVE(*lock) NO_LOCK_ANALYSIS {
   pthread_rwlock_wrlock(&lock->object);
 }
 
-static inline void rwlock_unlock(struct rwlock *lock)
+void rwlock_unlock(struct rwlock *lock)
     UNLOCKS(*lock) NO_LOCK_ANALYSIS {
   pthread_rwlock_unlock(&lock->object);
 }

wasmtime-wasi/sandboxed-system-primitives/src/posix.c

@@ -243,10 +243,6 @@ __wasi_errno_t wasmtime_ssp_clock_time_get(
   return 0;
 }
 
-struct fd_prestat {
-  const char *dir;
-};
-
 void fd_prestats_init(
     struct fd_prestats *pt
 ) {
@@ -653,32 +649,6 @@ static __wasi_errno_t fd_table_insert_fd(
   return fd_table_insert(ft, fo, rights_base, rights_inheriting, out);
 }
 
-__wasi_errno_t wasmtime_ssp_fd_prestat_get(
-#if !defined(WASMTIME_SSP_STATIC_CURFDS)
-    struct fd_prestats *prestats,
-#endif
-    __wasi_fd_t fd,
-    __wasi_prestat_t *buf
-) {
-  rwlock_rdlock(&prestats->lock);
-  struct fd_prestat *prestat;
-  __wasi_errno_t error = fd_prestats_get_entry(prestats, fd, &prestat);
-  if (error != 0) {
-    rwlock_unlock(&prestats->lock);
-    return error;
-  }
-
-  *buf = (__wasi_prestat_t) {
-    .pr_type = __WASI_PREOPENTYPE_DIR,
-  };
-
-  buf->u.dir.pr_name_len = strlen(prestat->dir);
-
-  rwlock_unlock(&prestats->lock);
-
-  return 0;
-}
-
 __wasi_errno_t wasmtime_ssp_fd_prestat_dir_name(
 #if !defined(WASMTIME_SSP_STATIC_CURFDS)
     struct fd_prestats *prestats,

wasmtime-wasi/sandboxed-system-primitives/src/posix.h

@@ -18,9 +18,12 @@
 #include "locking.h"
 
 struct fd_entry;
-struct fd_prestat;
 struct syscalls;
 
+struct fd_prestat {
+  const char *dir;
+};
+
 struct fd_table {
   struct rwlock lock;
   struct fd_entry *entries;

wasmtime-wasi/src/host_impls.rs

@@ -88,10 +88,66 @@ fn convert_errno(error: Errno) -> host::__wasi_errno_t {
     }
 }
 
+fn fd_prestats_get_entry(
+    pt: &host::fd_prestats,
+    fd: host::__wasi_fd_t,
+) -> Option<&host::fd_prestat> {
+    // Test for file descriptor existence
+    if fd as usize >= pt.size {
+        return None;
+    }
+
+    let prestat = unsafe { &*pt.prestats.add(fd as usize) };
+    if prestat.dir == ::std::ptr::null() {
+        return None;
+    }
+
+    Some(prestat)
+}
+
+macro_rules! rwlock_rdlock {
+    ($prestats:expr) => {
+        unsafe {
+            host::rwlock_rdlock(&mut (*$prestats).lock as *mut host::rwlock);
+        }
+    };
+}
+
+macro_rules! rwlock_unlock {
+    ($prestats:expr) => {
+        unsafe {
+            host::rwlock_unlock(&mut (*$prestats).lock as *mut host::rwlock);
+        }
+    };
+}
+
 pub fn wasmtime_ssp_proc_exit(rval: wasm32::__wasi_exitcode_t) {
     ::std::process::exit(rval as i32)
 }
 
+pub fn wasmtime_ssp_fd_prestat_get(
+    prestats: &mut host::fd_prestats,
+    fd: host::__wasi_fd_t,
+    buf: &mut host::__wasi_prestat_t,
+) -> host::__wasi_errno_t {
+    rwlock_rdlock!(prestats);
+
+    let ret_code = if let Some(prestat) = fd_prestats_get_entry(prestats, fd) {
+        (*buf).pr_type = host::__WASI_PREOPENTYPE_DIR as host::__wasi_preopentype_t;
+        unsafe {
+            let dir_name = ::std::ffi::CStr::from_ptr((*prestat).dir).to_str().unwrap();
+            (*buf).u.dir.pr_name_len = dir_name.len();
+        }
+        host::__WASI_ESUCCESS
+    } else {
+        host::__WASI_EBADF
+    };
+
+    rwlock_unlock!(prestats);
+
+    ret_code
+}
+
 pub fn wasmtime_ssp_sched_yield() -> host::__wasi_errno_t {
     unsafe {
         if libc::sched_yield() < 0 {

wasmtime-wasi/src/syscalls.rs

@@ -380,7 +380,7 @@ syscalls! {
             return return_encoded_errno(e);
         }
 
-        let e = host::wasmtime_ssp_fd_prestat_get(prestats, fd, &mut host_buf);
+        let e = host_impls::wasmtime_ssp_fd_prestat_get(&mut *prestats, fd, &mut host_buf);
 
         encode_prestat_byref(vmctx, buf, host_buf);