Fix ReadMagicAndReset.

ReadMagicAndReset is a function that is supposed to read a uint32_t
from the beginning of a file. However, it cannot handle the case where
the file offset of the fd is non-zero. Specifically, when an fd is
passed from dex2oat, dex2oat has probably read that file, causing the
file offset to be non-zero. This CL fixes the problem by resetting the
file offest before reading from the fd. Ideally, we should use `pread`,
but `ReadMagicAndReset` is part of `libartbase`, which has to support
Windows build where `pread` is not supported.

This change fixes odrefresh, which invokes dex2oat with fds.

Bug: 213446666
Test: atest odsign_e2e_tests
Change-Id: Ib5dd8e2855a4f0a391afa5313cf3a2d12f1a3121

1 files changed, 4 insertions, 0 deletions

diff --git a/libartbase/base/file_magic.cc b/libartbase/base/file_magic.cc
index 1471c59b73..21b762f6e0 100644
--- a/libartbase/base/file_magic.cc
+++ b/libartbase/base/file_magic.cc
@@ -44,6 +44,10 @@ File OpenAndReadMagic(const char* filename, uint32_t* magic, std::string* error_
 }
 
 bool ReadMagicAndReset(int fd, uint32_t* magic, std::string* error_msg) {
+  if (lseek(fd, 0, SEEK_SET) != 0) {
+    *error_msg = StringPrintf("Failed to seek to beginning of file : %s", strerror(errno));
+    return false;
+  }
   int n = TEMP_FAILURE_RETRY(read(fd, magic, sizeof(*magic)));
   if (n != sizeof(*magic)) {
     *error_msg = StringPrintf("Failed to find magic");