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[ELF] adjustOutputSections: don't copy SHF_EXECINSTR when an output does not contain input sections #70911

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Merged
merged 1 commit into from
Nov 2, 2023
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[ELF] adjustOutputSections: don't copy SHF_EXECINSTR when an output does not contain input sections #70911

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8 changes: 5 additions & 3 deletions lld/ELF/LinkerScript.cpp
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Expand Up @@ -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;
Expand All @@ -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 =
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Could be worth updating the comment on line 1170

  // 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.

Perhaps
"// We do not propagate SHF_EXECINSTR as in some cases this can lead to executable writeable SHT_NOBITS section."

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Thanks for the suggestion. Reworded a bit

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
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10 changes: 10 additions & 0 deletions lld/docs/ELF/linker_script.rst
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Expand Up @@ -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
-------------------

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1 change: 0 additions & 1 deletion lld/test/ELF/linkerscript/empty-synthetic-removed-flags.s
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Expand Up @@ -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"
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13 changes: 7 additions & 6 deletions lld/test/ELF/linkerscript/extend-pt-load2.test
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Expand Up @@ -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
11 changes: 6 additions & 5 deletions lld/test/ELF/linkerscript/insert-before.test
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Expand Up @@ -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

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14 changes: 7 additions & 7 deletions lld/test/ELF/linkerscript/lma-align2.test
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Expand Up @@ -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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