[llvm] 1f3d452 - [JITLink][AArch64] Use R-X permissions for the GOT.

[Lang Hames via llvm-commits]

Author: Lang Hames
Date: 2022-01-12T18:03:58+11:00
New Revision: 1f3d4525d87e9cd69c18098449a21ed0199fc459

URL: https://github.com/llvm/llvm-project/commit/1f3d4525d87e9cd69c18098449a21ed0199fc459
DIFF: https://github.com/llvm/llvm-project/commit/1f3d4525d87e9cd69c18098449a21ed0199fc459.diff

LOG: [JITLink][AArch64] Use R-X permissions for the GOT.

This consistent with ld64's treatment of the GOT, but the main aim here is a
short-term workaround for a bad interaction between stub code sequences and
memory layout: Stubs use LDRLiteral19 relocations to reference the GOT, but
BasicLayout currently puts RW- segments between R-- and R-X segments -- a large
RW- segment (or a large R-- for that matter) can cause the relocation to fail
with an out-of-range error.

Putting the GOT in R-X fixes this efficiently in practice. A more robust fix
will be to use a longer code sequence to materialize the GOT pointer and then
rewrite the stub to use a shorter sequence where possible.

Added: 
    llvm/test/ExecutionEngine/JITLink/AArch64/MachO_arm64_stub_to_got_distance_stress_test.s

Modified: 
    llvm/lib/ExecutionEngine/JITLink/MachO_arm64.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/ExecutionEngine/JITLink/MachO_arm64.cpp b/llvm/lib/ExecutionEngine/JITLink/MachO_arm64.cpp
index 844e76ab05425..45e70af642906 100644
--- a/llvm/lib/ExecutionEngine/JITLink/MachO_arm64.cpp
+++ b/llvm/lib/ExecutionEngine/JITLink/MachO_arm64.cpp
@@ -475,7 +475,7 @@ class PerGraphGOTAndPLTStubsBuilder_MachO_arm64
 private:
   Section &getGOTSection() {
     if (!GOTSection)
-      GOTSection = &G.createSection("$__GOT", MemProt::Read);
+      GOTSection = &G.createSection("$__GOT", MemProt::Read | MemProt::Exec);
     return *GOTSection;
   }
 

diff  --git a/llvm/test/ExecutionEngine/JITLink/AArch64/MachO_arm64_stub_to_got_distance_stress_test.s b/llvm/test/ExecutionEngine/JITLink/AArch64/MachO_arm64_stub_to_got_distance_stress_test.s
new file mode 100644
index 0000000000000..d9a8cae9d5005
--- /dev/null
+++ b/llvm/test/ExecutionEngine/JITLink/AArch64/MachO_arm64_stub_to_got_distance_stress_test.s
@@ -0,0 +1,17 @@
+# RUN: llvm-mc -triple=arm64-apple-darwin19 -filetype=obj -o %t.o %s
+# RUN: llvm-jitlink -noexec -phony-externals %t.o
+#
+# Use RW- empty space sufficient to push the R-- and R-X segments more than
+# 2^20 bytes apart. This will cause the LDRLiteral19 relocations from the STUB
+# section to the GOT to overflow if not handled correctly.
+
+        .section	__TEXT,__text,regular,pure_instructions
+	.ios_version_min 7, 0	sdk_version 16, 0
+	.globl	_main
+	.p2align	2
+_main:
+	b	_foo
+
+	.comm	_empty_space,2097152,0
+
+.subsections_via_symbols