[PATCH] libsanitizer: asan: Always skip first object from dl_iterate_phdr

[Michael Forney]

All platforms return the main executable as the first dl_phdr_info.
FreeBSD, NetBSD, Solaris, and Linux-musl place the executable name
in the dlpi_name field of this entry. It appears that only Linux-glibc
uses the empty string.

To make this work generically on all platforms, unconditionally
skip the first object (like is currently done for FreeBSD and NetBSD).
This fixes first DSO detection on Linux-musl. It also would likely
fix detection on Solaris/Illumos if it were to gain PIE support
(since dlpi_addr would not be NULL).

Additionally, only skip the Linux vDSO on Linux.

Finally, use the empty string as the "seen first dl_phdr_info"
marker rather than (char *)-1. If there was no other object, we
would try to dereference it for a string comparison.

Cherry-picked from upstream commit 795b07f5498c.

libsanitizer/

        * asan/asan_linux.cpp: Always skip first object from
        dl_iterate_phdr.
---
Is it possible that this change might make gcc 12? It fixes asan
on musl (without setting ASAN_OPTIONS=verify_asan_link_order=false),
which would be quite nice since this will be the first release that
libsanitizer works on musl.

 libsanitizer/asan/asan_linux.cpp | 30 ++++++++++++------------------
 1 file changed, 12 insertions(+), 18 deletions(-)

diff --git a/libsanitizer/asan/asan_linux.cpp b/libsanitizer/asan/asan_linux.cpp
index ad3693d5e6a..89ee48db7a2 100644
--- a/libsanitizer/asan/asan_linux.cpp
+++ b/libsanitizer/asan/asan_linux.cpp
@@ -131,30 +131,24 @@ static int FindFirstDSOCallback(struct dl_phdr_info 
*info, size_t size,
   VReport(2, "info->dlpi_name = %s\tinfo->dlpi_addr = %p\n", info->dlpi_name,
           (void *)info->dlpi_addr);
 
-  // Continue until the first dynamic library is found
-  if (!info->dlpi_name || info->dlpi_name[0] == 0)
-    return 0;
-
-  // Ignore vDSO
-  if (internal_strncmp(info->dlpi_name, "linux-", sizeof("linux-") - 1) == 0)
-    return 0;
+  const char **name = (const char **)data;
 
-#if SANITIZER_FREEBSD || SANITIZER_NETBSD
   // Ignore first entry (the main program)
-  char **p = (char **)data;
-  if (!(*p)) {
-    *p = (char *)-1;
+  if (!*name) {
+    *name = "";
     return 0;
   }
-#endif
 
-#if SANITIZER_SOLARIS
-  // Ignore executable on Solaris
-  if (info->dlpi_addr == 0)
+#    if SANITIZER_LINUX
+  // Ignore vDSO. glibc versions earlier than 2.15 (and some patched
+  // by distributors) return an empty name for the vDSO entry, so
+  // detect this as well.
+  if (!info->dlpi_name[0] ||
+      internal_strncmp(info->dlpi_name, "linux-", sizeof("linux-") - 1) == 0)
     return 0;
-#endif
+#    endif
 
-  *(const char **)data = info->dlpi_name;
+  *name = info->dlpi_name;
   return 1;
 }
 
@@ -175,7 +169,7 @@ void AsanCheckDynamicRTPrereqs() {
   // Ensure that dynamic RT is the first DSO in the list
   const char *first_dso_name = nullptr;
   dl_iterate_phdr(FindFirstDSOCallback, &first_dso_name);
-  if (first_dso_name && !IsDynamicRTName(first_dso_name)) {
+  if (first_dso_name && first_dso_name[0] && !IsDynamicRTName(first_dso_name)) 
{
     Report("ASan runtime does not come first in initial library list; "
            "you should either link runtime to your application or "
            "manually preload it with LD_PRELOAD.\n");
-- 
2.35.1