From 988adcc60dd671e4b75627412d09efb6ec88914f Mon Sep 17 00:00:00 2001
From: Julian Lettner <julian.lettner@apple.com>
Date: Wed, 4 Dec 2019 15:32:32 -0800
Subject: [PATCH] Fix `ignore_noninstrumented_modules` on Linux

My initial implementation of `ignore_noninstrumented_modules` for Linux
(landed in 61592420d3e2a896ff64b6bcd9728dd359d7d479) did not consider
that the GNU symbol table could be present but empty.  "Empty" means:
void of real symbol entries, but potential "terminator/placeholder"
symbols that are skipped via `header->symoffset`.  In this case
`last_symbol` is zero and we should avoid computing
`chains[last_symbol - header->symoffset]`.

This bug seems to only manifest in combination with `LD_PRELOAD`
(counterpart of `DYLD_INSERT_LIBRARIES` for Linux) and for small
binaries, (e.g., the "not" utility from the llvm test suite) that have
segments without any real symbols.

This should fix the remaining failing ASan tests on the Swift Linux CI.

rdar://57566645
---
 compiler-rt/lib/sanitizer_common/sanitizer_linux_libcdep.cc | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/compiler-rt/lib/sanitizer_common/sanitizer_linux_libcdep.cc b/compiler-rt/lib/sanitizer_common/sanitizer_linux_libcdep.cc
index b53cb5912a6d3..30a1b9865f8f7 100644
--- a/compiler-rt/lib/sanitizer_common/sanitizer_linux_libcdep.cc
+++ b/compiler-rt/lib/sanitizer_common/sanitizer_linux_libcdep.cc
@@ -629,6 +629,10 @@ class DynamicSegment {
       last_symbol = Max(buckets[i], last_symbol);
     }
 
+    if (last_symbol < header->symoffset) {
+      return header->symoffset;
+    }
+
     // Walk the bucket's chain to add the chain length to the total.
     uint32_t chain_entry;
     do {