From 8bb58fb4286ad03a243e32e31d67dd2bb5c117ef Mon Sep 17 00:00:00 2001
From: LemonBoy <thatlemon@gmail.com>
Date: Tue, 11 May 2021 15:49:53 +0200
Subject: [PATCH] std: Fix offset param splitting for preadv/pwritev

The kernel interface expects the offset as a low+high pair even on BE
systems.
---
 lib/std/os/linux.zig | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/lib/std/os/linux.zig b/lib/std/os/linux.zig
index d5e25f2ab7..fbb44ba7bd 100644
--- a/lib/std/os/linux.zig
+++ b/lib/std/os/linux.zig
@@ -59,6 +59,20 @@ const require_aligned_register_pair =
     std.Target.current.cpu.arch.isThumb();
 
 // Split a 64bit value into a {LSB,MSB} pair.
+// The LE/BE variants specify the endianness to assume.
+fn splitValueLE64(val: i64) [2]u32 {
+    const u = @bitCast(u64, val);
+    return [2]u32{
+        @truncate(u32, u),
+        @truncate(u32, u >> 32),
+    };
+}
+fn splitValueBE64(val: i64) [2]u32 {
+    return [2]u32{
+        @truncate(u32, u >> 32),
+        @truncate(u32, u),
+    };
+}
 fn splitValue64(val: i64) [2]u32 {
     const u = @bitCast(u64, val);
     switch (builtin.endian) {
@@ -310,7 +324,7 @@ pub fn read(fd: i32, buf: [*]u8, count: usize) usize {
 }
 
 pub fn preadv(fd: i32, iov: [*]const iovec, count: usize, offset: i64) usize {
-    const offset_halves = splitValue64(offset);
+    const offset_halves = splitValueLE64(offset);
     return syscall5(
         .preadv,
         @bitCast(usize, @as(isize, fd)),
@@ -343,7 +357,7 @@ pub fn writev(fd: i32, iov: [*]const iovec_const, count: usize) usize {
 }
 
 pub fn pwritev(fd: i32, iov: [*]const iovec_const, count: usize, offset: i64) usize {
-    const offset_halves = splitValue64(offset);
+    const offset_halves = splitValueLE64(offset);
     return syscall5(
         .pwritev,
         @bitCast(usize, @as(isize, fd)),