util_unix: Move dependency on libc's errno to callers.

Make progress on reoving the libc dependency from util_unix, so we
can remove the libc dependency for x86_64-unknown-linux-none.

Author: briansmith

src/getrandom.rs

@@ -20,6 +20,7 @@ use core::{ffi::c_void, mem::MaybeUninit};
 
 pub fn getrandom_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
     sys_fill_exact(dest, |buf| unsafe {
-        libc::getrandom(buf.as_mut_ptr().cast::<c_void>(), buf.len(), 0)
+        let ret: isize = libc::getrandom(buf.as_mut_ptr().cast::<c_void>(), buf.len(), 0);
+        usize::try_from(ret).map_err(|_| last_os_error())
     })
 }

src/linux_android.rs

@@ -7,13 +7,17 @@ pub fn getrandom_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
 }
 
 // Also used by linux_android_with_fallback to check if the syscall is available.
-pub fn getrandom_syscall(buf: &mut [MaybeUninit<u8>]) -> libc::ssize_t {
-    unsafe {
+pub fn getrandom_syscall(buf: &mut [MaybeUninit<u8>]) -> Result<usize, Error> {
+    use crate::util_libc::last_os_error;
+
+    let ret: libc::c_long = unsafe {
         libc::syscall(
             libc::SYS_getrandom,
             buf.as_mut_ptr().cast::<core::ffi::c_void>(),
             buf.len(),
             0,
-        ) as libc::ssize_t
-    }
+        )
+    };
+    const _: () = assert!(core::mem::size_of::<libc::c_long>() == core::mem::size_of::<isize>());
+    usize::try_from(ret as isize).map_err(|_| last_os_error())
 }

src/linux_android_with_fallback.rs

@@ -1,5 +1,5 @@
 //! Implementation for Linux / Android with `/dev/urandom` fallback
-use crate::{lazy::LazyBool, linux_android, use_file, util_libc::last_os_error, Error};
+use crate::{lazy::LazyBool, linux_android, use_file, Error};
 use core::mem::MaybeUninit;
 
 pub fn getrandom_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
@@ -13,17 +13,13 @@ pub fn getrandom_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
 }
 
 fn is_getrandom_available() -> bool {
-    if linux_android::getrandom_syscall(&mut []) < 0 {
-        match last_os_error().raw_os_error() {
-            Some(libc::ENOSYS) => false, // No kernel support
-            // The fallback on EPERM is intentionally not done on Android since this workaround
-            // seems to be needed only for specific Linux-based products that aren't based
-            // on Android. See https://github.com/rust-random/getrandom/issues/229.
-            #[cfg(target_os = "linux")]
-            Some(libc::EPERM) => false, // Blocked by seccomp
-            _ => true,
-        }
-    } else {
-        true
+    match linux_android::getrandom_syscall(&mut []) {
+        Err(err) if err.raw_os_error() == Some(libc::ENOSYS) => false, // No kernel support
+        // The fallback on EPERM is intentionally not done on Android since this workaround
+        // seems to be needed only for specific Linux-based products that aren't based
+        // on Android. See https://github.com/rust-random/getrandom/issues/229.
+        #[cfg(target_os = "linux")]
+        Err(err) if err.raw_os_error() == Some(libc::EPERM) => false, // Blocked by seccomp
+        _ => true,
     }
 }

src/netbsd.rs

@@ -2,7 +2,7 @@
 use crate::{lazy::LazyPtr, util_unix::sys_fill_exact, Error};
 use core::{ffi::c_void, mem::MaybeUninit, ptr};
 
-fn kern_arnd(buf: &mut [MaybeUninit<u8>]) -> libc::ssize_t {
+fn kern_arnd(buf: &mut [MaybeUninit<u8>]) -> Result<usize, Error> {
     static MIB: [libc::c_int; 2] = [libc::CTL_KERN, libc::KERN_ARND];
     let mut len = buf.len();
     let ret = unsafe {
@@ -16,9 +16,9 @@ fn kern_arnd(buf: &mut [MaybeUninit<u8>]) -> libc::ssize_t {
         )
     };
     if ret == -1 {
-        -1
+        Err(last_os_error())
     } else {
-        len as libc::ssize_t
+        Ok(len)
     }
 }
 
@@ -38,7 +38,8 @@ pub fn getrandom_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
     if !fptr.is_null() {
         let func: GetRandomFn = unsafe { core::mem::transmute(fptr) };
         return sys_fill_exact(dest, |buf| unsafe {
-            func(buf.as_mut_ptr().cast::<u8>(), buf.len(), 0)
+            let ret: isize = func(buf.as_mut_ptr().cast::<u8>(), buf.len(), 0);
+            usize::try_from(ret as isize).map_err(|_| last_os_error())
         });
     }
 

src/use_file.rs

@@ -1,5 +1,9 @@
 //! Implementations that just need to read from a file
-use crate::{util_libc::open_readonly, util_unix::sys_fill_exact, Error};
+use crate::{
+    util_libc::{last_os_error, open_readonly},
+    util_unix::sys_fill_exact,
+    Error,
+};
 use core::{
     cell::UnsafeCell,
     ffi::c_void,
@@ -22,7 +26,8 @@ const FD_UNINIT: usize = usize::max_value();
 pub fn getrandom_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
     let fd = get_rng_fd()?;
     sys_fill_exact(dest, |buf| unsafe {
-        libc::read(fd, buf.as_mut_ptr().cast::<c_void>(), buf.len())
+        let ret: isize = libc::read(fd, buf.as_mut_ptr().cast::<c_void>(), buf.len());
+        usize::try_from(ret).map_err(|_| last_os_error())
     })
 }
 

src/util_unix.rs

@@ -1,20 +1,18 @@
 #![allow(dead_code)]
-use crate::{util_libc::last_os_error, Error};
+use crate::Error;
 use core::mem::MaybeUninit;
 
-// Fill a buffer by repeatedly invoking a system call. The `sys_fill` function:
-//   - should return -1 and set errno on failure
-//   - should return the number of bytes written on success
+// Fill a buffer by repeatedly invoking a system call. The `sys_fill` function
+// must return `Ok(written)` where `written` is the number of bytes written,
+// or otherwise an error.
 pub fn sys_fill_exact(
     mut buf: &mut [MaybeUninit<u8>],
-    sys_fill: impl Fn(&mut [MaybeUninit<u8>]) -> libc::ssize_t,
+    sys_fill: impl Fn(&mut [MaybeUninit<u8>]) -> Result<usize, Error>,
 ) -> Result<(), Error> {
     while !buf.is_empty() {
-        let res = sys_fill(buf);
-        match res {
-            res if res > 0 => buf = buf.get_mut(res as usize..).ok_or(Error::UNEXPECTED)?,
-            -1 => {
-                let err = last_os_error();
+        match sys_fill(buf) {
+            Ok(res) if res > 0 => buf = buf.get_mut(res..).ok_or(Error::UNEXPECTED)?,
+            Err(err) => {
                 // We should try again if the call was interrupted.
                 if err.raw_os_error() != Some(libc::EINTR) {
                     return Err(err);