[ELFAttributeParser] Skip unknown vendor subsections.

An .ARM.attributes section is divided into subsections, each labelled
with a vendor name. There is one standardised vendor name, which must
be used for all attributes that affect compatibility. Subsections
labelled with other vendor names can be used for optimisation
purposes, but it has to be safe for an object file consumer to ignore
them if it doesn't recognise the vendor name.

LLD currently terminates parsing of the whole attributes section as
soon as it encounters a subsection with a vendor name it doesn't
recognise (which is anything other than the standard one). This can
prevent it from detecting compatibility issues, if a standard
subsection followed the vendor-specific one.

This patch modifies the attribute parser so that unrecognised vendor
subsections are silently skipped, and the subsections beyond them are
still processed.

Differential Revision: https://reviews.llvm.org/D153335

Author: statham-arm

lld/test/ELF/arm-tag-vfp-args-errs.s

@@ -1,16 +1,25 @@
 // REQUIRES:arm
-// RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %S/Inputs/arm-vfp-arg-base.s -o %tbase.o
-// RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %S/Inputs/arm-vfp-arg-vfp.s -o %tvfp.o
-// RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %S/Inputs/arm-vfp-arg-toolchain.s -o %ttoolchain.o
-// RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %s -o %t.o
-// RUN: not ld.lld %t.o %tbase.o %tvfp.o -o%t 2>&1 | FileCheck %s
-// RUN: not ld.lld %t.o %tbase.o %ttoolchain.o -o%t 2>&1 | FileCheck %s
-// RUN: not ld.lld %t.o %tvfp.o %tbase.o -o%t 2>&1 | FileCheck %s
-// RUN: not ld.lld %t.o %tvfp.o %ttoolchain.o -o%t 2>&1 | FileCheck %s
-// RUN: not ld.lld %t.o %ttoolchain.o %tbase.o -o%t 2>&1 | FileCheck %s
-// RUN: not ld.lld %t.o %ttoolchain.o %tvfp.o -o%t 2>&1 | FileCheck %s
+
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+
+// RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %S/Inputs/arm-vfp-arg-base.s -o %t/base.o
+// RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %S/Inputs/arm-vfp-arg-vfp.s -o %t/vfp.o
+// RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %S/Inputs/arm-vfp-arg-toolchain.s -o %t/toolchain.o
+// RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %t/main.s -o %t/main.o
+// RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %t/vendor.s -o %t/vendor.o
+// RUN: not ld.lld %t/main.o %t/base.o %t/vfp.o -o%t/a.out 2>&1 | FileCheck %s
+// RUN: not ld.lld %t/main.o %t/base.o %t/vendor.o -o%t/a.out 2>&1 | FileCheck %s
+// RUN: not ld.lld %t/main.o %t/base.o %t/toolchain.o -o%t/a.out 2>&1 | FileCheck %s
+// RUN: not ld.lld %t/main.o %t/vfp.o %t/base.o -o%t/a.out 2>&1 | FileCheck %s
+// RUN: not ld.lld %t/main.o %t/vfp.o %t/toolchain.o -o%t/a.out 2>&1 | FileCheck %s
+// RUN: not ld.lld %t/main.o %t/toolchain.o %t/base.o -o%t/a.out 2>&1 | FileCheck %s
+// RUN: not ld.lld %t/main.o %t/toolchain.o %t/vfp.o -o%t/a.out 2>&1 | FileCheck %s
 
 // CHECK: incompatible Tag_ABI_VFP_args
+
+//--- main.s
+
 	.arch armv7-a
 	.eabi_attribute 20, 1
 	.eabi_attribute 21, 1
@@ -27,3 +36,52 @@
         .globl _start
         .type _start, %function
 _start: bx lr
+
+//--- vendor.s
+
+        .syntax unified
+
+        // Manually construct a custom .ARM.attributes section
+        .section .ARM.attributes,"",%0x70000003 // SHT_ARM_ATTRIBUTES
+
+        // Initial byte giving the section format version
+        .byte 'A'
+
+        // Subsection with a name that won't be recognised as a known vendor
+vendor_subsect_start:
+        .word vendor_subsect_end - vendor_subsect_start // subsection length
+        .asciz "ShouldBeIgnored" // vendor name
+        .dcb.b 64, 0xff // dummy vendor section contents
+vendor_subsect_end:
+
+        // Subsection that should be identical to the attributes defined by
+        // Inputs/arm-vfp-arg-vfp.s
+aeabi_subsect_start:
+        .word aeabi_subsect_end - aeabi_subsect_start
+        .asciz "aeabi" // vendor name indicating the standard subsection
+file_subsubsect_start:
+        .byte 1 // introduce sub-subsection of attributes for the whole file
+        .word file_subsubsect_end - file_subsubsect_start // sub-subsection len
+        .byte 5 // CPU_name
+        .asciz "7-A"
+        .byte 6, 10 // CPU_arch = ARM v7
+        .byte 7, 'A' // CPU_arch_profile = Application
+        .byte 8, 1 // ARM_ISA_use = Permitted
+        .byte 9, 2 // THUMB_ISA_use = Thumb-2
+        .byte 18, 4 // ABI_PCS_wchar_t = 4-byte
+        .byte 20, 1 // ABI_FP_denormal = IEEE-754
+        .byte 21, 1 // ABI_FP_exceptions = IEEE-754
+        .byte 23, 3 // ABI_FP_number_model = IEEE-754
+        .byte 24, 1 // ABI_align_needed = 8-byte alignment
+        .byte 25, 1 // ABI_align_preserved = 8-byte data alignment
+        .byte 26, 2 // ABI_enum_size = Int32
+        .byte 28, 1 // ABI_VFP_args = AAPCS VFP
+        .byte 30, 6 // ABI_optimization_goals = Best Debugging
+        .byte 34, 1 // CPU_unaligned_access = v6-style
+file_subsubsect_end:
+aeabi_subsect_end:
+
+        .text
+        .global f1
+        .type f1, %function
+f1:     bx lr

llvm/lib/Support/ELFAttributeParser.cpp

@@ -127,10 +127,14 @@ Error ELFAttributeParser::parseSubsection(uint32_t length) {
     sw->printString("Vendor", vendorName);
   }
 
-  // Ignore unrecognized vendor-name.
-  if (vendorName.lower() != vendor)
-    return createStringError(errc::invalid_argument,
-                             "unrecognized vendor-name: " + vendorName);
+  // Handle a subsection with an unrecognized vendor-name by skipping
+  // over it to the next subsection. ADDENDA32 in the Arm ABI defines
+  // that vendor attribute sections must not affect compatibility, so
+  // this should always be safe.
+  if (vendorName.lower() != vendor) {
+    cursor.seek(end);
+    return Error::success();
+  }
 
   while (cursor.tell() < end) {
     /// Tag_File | Tag_Section | Tag_Symbol   uleb128:byte-size