[lld] a40f651 - [ELF] adjustOutputSections: don't copy SHF_EXECINSTR when an output does not contain input sections (#70911)
via llvm-commits
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Wed Nov 1 22:35:32 PDT 2023
Author: Fangrui Song
Date: 2023-11-01T22:35:28-07:00
New Revision: a40f651a065ef28fee17bc1f22cce08247eff950
URL: https://github.com/llvm/llvm-project/commit/a40f651a065ef28fee17bc1f22cce08247eff950
DIFF: https://github.com/llvm/llvm-project/commit/a40f651a065ef28fee17bc1f22cce08247eff950.diff
LOG: [ELF] adjustOutputSections: don't copy SHF_EXECINSTR when an output does not contain input sections (#70911)
For an output section with no input section, GNU ld eliminates the
output section when there are only symbol assignments (e.g.
`.foo : { symbol = 42; }`) but not for `.foo : { . += 42; }`
(`SHF_ALLOC|SHF_WRITE`).
We choose to retain such an output section with a symbol assignment
(unless unreferenced `PROVIDE`). We copy the previous section flag (see
https://reviews.llvm.org/D37736) to hopefully make the current PT_LOAD
segment extend to the current output section:
* decrease the number of PT_LOAD segments
* If a new PT_LOAD segment is introduced without a page-size
alignment as a separator, there may be a run-time crash.
However, this `flags` copying behavior is not suitable for
`.foo : { . += 42; }` when `flags` contains `SHF_EXECINSTR`. The
executable bit is surprising
(https://discourse.llvm.org/t/lld-output-section-flag-assignment-behavior/74359).
I think we should drop SHF_EXECINSTR when copying `flags`. The risk is a
code section followed by `.foo : { symbol = 42; }` will be broken, which
I believe is unrelated as such uses are almost always related to data
sections.
For data-command-only output sections (e.g. `.foo : { QUAD(42) }`), we
keep allowing copyable SHF_WRITE.
Some tests are updated to drop the SHF_EXECINSTR flag. GNU ld doesn't
set SHF_EXECINSTR as well, though it sets SHF_WRITE for some tests while
we don't.
Added:
Modified:
lld/ELF/LinkerScript.cpp
lld/docs/ELF/linker_script.rst
lld/test/ELF/linkerscript/empty-synthetic-removed-flags.s
lld/test/ELF/linkerscript/extend-pt-load2.test
lld/test/ELF/linkerscript/insert-before.test
lld/test/ELF/linkerscript/lma-align2.test
Removed:
################################################################################
diff --git a/lld/ELF/LinkerScript.cpp b/lld/ELF/LinkerScript.cpp
index df091613dc0a144..1b8acbe1c908b1d 100644
--- a/lld/ELF/LinkerScript.cpp
+++ b/lld/ELF/LinkerScript.cpp
@@ -1167,7 +1167,9 @@ void LinkerScript::adjustOutputSections() {
// * The address assignment.
// The other option is to pick flags that minimize the impact the section
// will have on the rest of the linker. That is why we copy the flags from
- // the previous sections. Only a few flags are needed to keep the impact low.
+ // the previous sections. We copy just SHF_ALLOC and SHF_WRITE to keep the
+ // impact low. We do not propagate SHF_EXECINSTR as in some cases this can
+ // lead to executable writeable section.
uint64_t flags = SHF_ALLOC;
SmallVector<StringRef, 0> defPhdrs;
@@ -1193,8 +1195,8 @@ void LinkerScript::adjustOutputSections() {
// We do not want to keep any special flags for output section
// in case it is empty.
if (isEmpty)
- sec->flags = flags & ((sec->nonAlloc ? 0 : (uint64_t)SHF_ALLOC) |
- SHF_WRITE | SHF_EXECINSTR);
+ sec->flags =
+ flags & ((sec->nonAlloc ? 0 : (uint64_t)SHF_ALLOC) | SHF_WRITE);
// The code below may remove empty output sections. We should save the
// specified program headers (if exist) and propagate them to subsequent
diff --git a/lld/docs/ELF/linker_script.rst b/lld/docs/ELF/linker_script.rst
index fbd96abcb44c5b1..3606ef4fe4b8ee1 100644
--- a/lld/docs/ELF/linker_script.rst
+++ b/lld/docs/ELF/linker_script.rst
@@ -97,6 +97,16 @@ The presence of ``address`` can cause the condition unsatisfied. LLD will warn.
GNU ld from Binutils 2.35 onwards will reduce sh_addralign so that
sh_addr=0 (modulo sh_addralign).
+When an output section has no input section, GNU ld will eliminate it if it
+only contains symbol assignments (e.g. ``.foo { symbol = 42; }``). LLD will
+retain such sections unless all the symbol assignments are unreferenced
+``PROVIDED``.
+
+When an output section has no input section but advances the location counter,
+GNU ld sets the ``SHF_WRITE`` flag. LLD sets the SHF_WRITE flag only if the
+preceding output section with non-empty input sections also has the SHF_WRITE
+flag.
+
Output section type
-------------------
diff --git a/lld/test/ELF/linkerscript/empty-synthetic-removed-flags.s b/lld/test/ELF/linkerscript/empty-synthetic-removed-flags.s
index b8d72702425f19f..dfc502e8da9e1d4 100644
--- a/lld/test/ELF/linkerscript/empty-synthetic-removed-flags.s
+++ b/lld/test/ELF/linkerscript/empty-synthetic-removed-flags.s
@@ -29,7 +29,6 @@
# EMPTY-NEXT: Type: SHT_PROGBITS
# EMPTY-NEXT: Flags [
# EMPTY-NEXT: SHF_ALLOC
-# EMPTY-NEXT: SHF_EXECINSTR
# EMPTY-NEXT: ]
.section .foo,"ax"
diff --git a/lld/test/ELF/linkerscript/extend-pt-load2.test b/lld/test/ELF/linkerscript/extend-pt-load2.test
index 3be94a5670d8cfe..6c0e3b2863f9133 100644
--- a/lld/test/ELF/linkerscript/extend-pt-load2.test
+++ b/lld/test/ELF/linkerscript/extend-pt-load2.test
@@ -19,12 +19,13 @@ SECTIONS {
.data.rel.ro : { *(.data.rel.ro) }
}
-# CHECK: .rodata PROGBITS 0000000000000215 000215 000001 00 A 0
-# CHECK-NEXT: foo PROGBITS 0000000000000216 000216 000000 00 A 0
-# CHECK-NEXT: .text PROGBITS 0000000000000218 000218 000001 00 AX 0
-# CHECK-NEXT: bar PROGBITS 0000000000000219 000219 000de7 00 AX 0
+# CHECK: .rodata PROGBITS 000000000000024d 00024d 000001 00 A 0
+# CHECK-NEXT: foo PROGBITS 000000000000024e 00024e 000000 00 A 0
+# CHECK-NEXT: .text PROGBITS 0000000000000250 000250 000001 00 AX 0
+# CHECK-NEXT: bar PROGBITS 0000000000000251 000251 000daf 00 A 0
# CHECK-NEXT: .data.rel.ro PROGBITS 0000000000001000 001000 000001 00 WA 0
-# CHECK: LOAD 0x000000 0x0000000000000000 0x0000000000000000 0x000216 0x000216 R 0x1000
-# CHECK: LOAD 0x000218 0x0000000000000218 0x0000000000000218 0x000de8 0x000de8 R E 0x1000
+# CHECK: LOAD 0x000000 0x0000000000000000 0x0000000000000000 0x00024e 0x00024e R 0x1000
+# CHECK-NEXT: LOAD 0x000250 0x0000000000000250 0x0000000000000250 0x000001 0x000001 R E 0x1000
+# CHECK-NEXT: LOAD 0x000251 0x0000000000000251 0x0000000000000251 0x000daf 0x000daf R 0x1000
# CHECK-NEXT: LOAD 0x001000 0x0000000000001000 0x0000000000001000 0x000068 0x000068 RW 0x1000
diff --git a/lld/test/ELF/linkerscript/insert-before.test b/lld/test/ELF/linkerscript/insert-before.test
index f9611538c013b2b..e6ed413639827a9 100644
--- a/lld/test/ELF/linkerscript/insert-before.test
+++ b/lld/test/ELF/linkerscript/insert-before.test
@@ -9,13 +9,14 @@
# RUN: llvm-readelf -S -l %t1 | FileCheck %s
# CHECK: Name Type Address Off Size ES Flg
# CHECK-NEXT: NULL
-# CHECK-NEXT: .foo.text PROGBITS 0000000000000000 001000 000008 00 AX
-# CHECK-NEXT: .text PROGBITS 0000000000000008 001008 000008 00 AX
-# CHECK-NEXT: .byte PROGBITS 0000000000000010 001010 000001 00 AX
-# CHECK-NEXT: .foo.data PROGBITS 0000000000000011 001011 000008 00 WA
-# CHECK-NEXT: .data PROGBITS 0000000000000019 001019 000008 00 WA
+# CHECK-NEXT: .foo.text PROGBITS 0000000000000000 001000 000008 00 AX 0
+# CHECK-NEXT: .text PROGBITS 0000000000000008 001008 000008 00 AX 0
+# CHECK-NEXT: .byte PROGBITS 0000000000000010 001010 000001 00 A 0
+# CHECK-NEXT: .foo.data PROGBITS 0000000000000011 001011 000008 00 WA 0
+# CHECK-NEXT: .data PROGBITS 0000000000000019 001019 000008 00 WA 0
# CHECK: Type
# CHECK-NEXT: LOAD {{.*}} R E
+# CHECK-NEXT: LOAD {{.*}} R
# CHECK-NEXT: LOAD {{.*}} RW
# CHECK-NEXT: GNU_STACK {{.*}} RW
diff --git a/lld/test/ELF/linkerscript/lma-align2.test b/lld/test/ELF/linkerscript/lma-align2.test
index 2177101ff37475d..23b06b179b6e85d 100644
--- a/lld/test/ELF/linkerscript/lma-align2.test
+++ b/lld/test/ELF/linkerscript/lma-align2.test
@@ -7,17 +7,17 @@
# CHECK: Name Type Address Off Size ES Flg Lk Inf Al
# CHECK-NEXT: NULL 0000000000000000 000000 000000 00 0 0 0
# CHECK-NEXT: .text PROGBITS 0000000008000000 001000 000001 00 AX 0 0 4
-# CHECK-NEXT: .data PROGBITS 0000000020000000 002000 000006 00 AX 0 0 8
-# CHECK-NEXT: .data2 PROGBITS 000000000800000e 00200e 000008 00 AX 0 0 1
-# CHECK-NEXT: .data3 PROGBITS 0000000020000008 002018 000000 00 AX 0 0 8
-# CHECK-NEXT: .data4 PROGBITS 0000000008000018 002018 000008 00 AX 0 0 1
+# CHECK-NEXT: .data PROGBITS 0000000020000000 002000 000006 00 A 0 0 8
+# CHECK-NEXT: .data2 PROGBITS 000000000800000e 00200e 000008 00 A 0 0 1
+# CHECK-NEXT: .data3 PROGBITS 0000000020000008 002018 000000 00 A 0 0 8
+# CHECK-NEXT: .data4 PROGBITS 0000000008000018 002018 000008 00 A 0 0 1
# CHECK: Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
# CHECK-NEXT: LOAD 0x001000 0x0000000008000000 0x0000000008000000 0x000001 0x000001 R E 0x1000
-# CHECK-NEXT: LOAD 0x002000 0x0000000020000000 0x0000000008000008 0x000006 0x000006 R E 0x1000
-# CHECK-NEXT: LOAD 0x00200e 0x000000000800000e 0x000000000800000e 0x000008 0x000008 R E 0x1000
-# CHECK-NEXT: LOAD 0x002018 0x0000000008000018 0x0000000008000018 0x000008 0x000008 R E 0x1000
+# CHECK-NEXT: LOAD 0x002000 0x0000000020000000 0x0000000008000008 0x000006 0x000006 R 0x1000
+# CHECK-NEXT: LOAD 0x00200e 0x000000000800000e 0x000000000800000e 0x000008 0x000008 R 0x1000
+# CHECK-NEXT: LOAD 0x002018 0x0000000008000018 0x0000000008000018 0x000008 0x000008 R 0x1000
MEMORY {
CODE (rx) : ORIGIN = 0x08000000, LENGTH = 100K
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