[asan] Always skip first object from dl_iterate_phdr

[mcf]

Summary

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 with clang -shared-libsan/-shared-libasan and GCC's
default. 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.

[MaskRay]

FWIW you can reuse the old differential D119515. You can click "Add Action"->"Reopen Revision", and then upload a new diff via web or via arc diff 'HEAD^'.
Reusing the old one may be preferred so that folks have better ideas what's tricky.

[mcf]

In D123344#3437672, @MaskRay wrote:

FWIW you can reuse the old differential D119515. You can click "Add Action"->"Reopen Revision", and then upload a new diff via web or via arc diff 'HEAD^'.
Reusing the old one may be preferred so that folks have better ideas what's tricky.

Ah, thanks for the tip. I tried to update it at first but it said I couldn't because it was closed. I didn't realize I could just reopen it. Closing this in favor of the old diff.

compiler-rt/lib/asan/asan_linux.cpp

 void AsanCheckDynamicRTPrereqs() {}
 void AsanCheckIncompatibleRT() {}
 #else
 static int FindFirstDSOCallback(struct dl_phdr_info *info, size_t size,
                                 void *data) {
   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. Some old versions of glibc (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;
 }
 
 static bool IsDynamicRTName(const char *libname) {
   return internal_strstr(libname, "libclang_rt.asan") ||
     internal_strstr(libname, "libasan.so");
 }
 
 static void ReportIncompatibleRT() {
   Report("Your application is linked against incompatible ASan runtimes.\n");
   Die();
 }
 
 void AsanCheckDynamicRTPrereqs() {
   if (!ASAN_DYNAMIC || !flags()->verify_asan_link_order)
     return;
 
   // 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");
     Die();
   }
 }
 
 void AsanCheckIncompatibleRT() {