Disable CFI for weakly linked syscalls

Currently, when enabling CFI via -Zsanitizer=cfi and executing e.g.
std::sys::random::getrandom, we can observe a CFI violation. This is
the case for all consumers of the std::sys::pal::weak::weak macro,
as it is defining weak functions which don't show up in LLVM IR
metadata. CFI fails for all these functions.

Similar to other such cases in
rust-lang#115199, this change stops
emitting the CFI typecheck for consumers of the macro via the
\#[no_sanitize(cfi)] attribute.

library/std/src/sys/fs/unix.rs

@@ -1454,6 +1454,18 @@ impl File {
         Ok(())
     }
 
+    #[cfg_attr(
+        any(
+            target_os = "android",
+            all(
+                target_os = "linux",
+                target_env = "gnu",
+                target_pointer_width = "32",
+                not(target_arch = "riscv32")
+            )
+        ),
+        no_sanitize(cfi)
+    )]
     pub fn set_times(&self, times: FileTimes) -> io::Result<()> {
         #[cfg(not(any(
             target_os = "redox",

library/std/src/sys/pal/unix/fd.rs

@@ -251,6 +251,7 @@ impl FileDesc {
     }
 
     #[cfg(all(target_os = "android", target_pointer_width = "32"))]
+    #[no_sanitize(cfi)]
     pub fn read_vectored_at(&self, bufs: &mut [IoSliceMut<'_>], offset: u64) -> io::Result<usize> {
         super::weak::weak!(fn preadv64(libc::c_int, *const libc::iovec, libc::c_int, off64_t) -> isize);
 

library/std/src/sys/pal/unix/process/process_unix.rs

@@ -434,6 +434,7 @@ impl Command {
         target_os = "nto",
         target_vendor = "apple",
     ))]
+    #[cfg_attr(target_os = "linux", no_sanitize(cfi))]
     fn posix_spawn(
         &mut self,
         stdio: &ChildPipes,

library/std/src/sys/pal/unix/thread.rs

@@ -188,6 +188,7 @@ impl Thread {
     }
 
     #[cfg(any(target_os = "solaris", target_os = "illumos", target_os = "nto"))]
+    #[no_sanitize(cfi)]
     pub fn set_name(name: &CStr) {
         weak! {
             fn pthread_setname_np(

library/std/src/sys/pal/unix/time.rs

@@ -96,6 +96,15 @@ impl Timespec {
         }
     }
 
+    #[cfg_attr(
+        all(
+            target_os = "linux",
+            target_env = "gnu",
+            target_pointer_width = "32",
+            not(target_arch = "riscv32")
+        ),
+        no_sanitize(cfi)
+    )]
     pub fn now(clock: libc::clockid_t) -> Timespec {
         use crate::mem::MaybeUninit;
         use crate::sys::cvt;

library/std/src/sys/pal/unix/weak.rs

@@ -144,6 +144,9 @@ unsafe fn fetch(name: &str) -> *mut libc::c_void {
 #[cfg(not(any(target_os = "linux", target_os = "android")))]
 pub(crate) macro syscall {
     (fn $name:ident($($arg_name:ident: $t:ty),*) -> $ret:ty) => (
+        // FIXME: Rust currently omits weak function definitions
+        // and its metadata from LLVM IR.
+        #[no_sanitize(cfi)]
         unsafe fn $name($($arg_name: $t),*) -> $ret {
             weak! { fn $name($($t),*) -> $ret }