[libc] Fix pread under msan (#80893)

The pread function wasn't properly unpoisoning its result under msan,
causing test failures downstream when I tried to roll it out. This patch
adds the msan unpoison call that fixes the issue.

Author: michaelrj-google

libc/src/unistd/linux/CMakeLists.txt

@@ -283,6 +283,7 @@ add_entrypoint_object(
     libc.include.unistd
     libc.include.sys_syscall
     libc.src.__support.OSUtil.osutil
+    libc.src.__support.macros.sanitizer
     libc.src.errno.errno
 )
 
@@ -309,6 +310,7 @@ add_entrypoint_object(
     libc.include.unistd
     libc.include.sys_syscall
     libc.src.__support.OSUtil.osutil
+    libc.src.__support.macros.sanitizer
     libc.src.errno.errno
 )
 

libc/src/unistd/linux/pread.cpp

@@ -10,7 +10,7 @@
 
 #include "src/__support/OSUtil/syscall.h" // For internal syscall function.
 #include "src/__support/common.h"
-
+#include "src/__support/macros/sanitizer.h" // for MSAN_UNPOISON
 #include "src/errno/libc_errno.h"
 #include <stdint.h>      // For uint64_t.
 #include <sys/syscall.h> // For syscall numbers.
@@ -28,6 +28,9 @@ LLVM_LIBC_FUNCTION(ssize_t, pread,
   ssize_t ret = LIBC_NAMESPACE::syscall_impl<ssize_t>(SYS_pread64, fd, buf,
                                                       count, offset);
 #endif
+  // The cast is important since there is a check that dereferences the pointer
+  // which fails on void*.
+  MSAN_UNPOISON(reinterpret_cast<char *>(buf), count);
   if (ret < 0) {
     libc_errno = static_cast<int>(-ret);
     return -1;