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Function symbol from assembler misses lowest bit set in Thumb mode #44205

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agners opened this issue Feb 9, 2020 · 4 comments
Closed

Function symbol from assembler misses lowest bit set in Thumb mode #44205

agners opened this issue Feb 9, 2020 · 4 comments
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backend:ARM bugzilla Issues migrated from bugzilla

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@agners
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agners commented Feb 9, 2020

Bugzilla Link 44860
Resolution FIXED
Resolved on Feb 24, 2021 14:22
Version 10.0
OS Linux
Blocks #4440
Attachments reproducer assembly
CC @jcai19,@DavidSpickett,@m-gupta,@nickdesaulniers,@smithp35

Extended Description

Using the .type directive to mark a label as a function in Thumb mode does not get the correct symbol table entry.

    .syntax unified
    .text
    .thumb

__setup_mmu:
it ne
blne __setup_mmu
.type __setup_mmu, %function // Move this line before __setup_mmu for correct behaviour.

llvm-mc --triple=armv7a-linux-gnueabihf blne.s -filetype=obj -o blne.o --arm-add-build-attributes
llvm-readelf --symbols blne.o

Symbol table '.symtab' contains 3 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 00000000 0 NOTYPE LOCAL DEFAULT UND
1: 00000000 0 NOTYPE LOCAL DEFAULT 2 $t.0
2: 00000000 0 FUNC LOCAL DEFAULT 2 __setup_mmu

This has been observed when trying to build Linux for 32-bit ARM in Thumb2 mode. When the object file gets linked, the linker inserts a blx instruction which switches the CPU instruction set...
@DavidSpickett
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Not working (label then .type)

$ ./llvm-mc --triple=armv7a-linux-gnueabihf /tmp/test.s -filetype=obj -o /tmp/test.o --arm-add-build-attributes
$ ./llvm-readelf --symbols /tmp/test.o

Symbol table '.symtab' contains 3 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 00000000 0 NOTYPE LOCAL DEFAULT UND
1: 00000000 0 NOTYPE LOCAL DEFAULT 2 $t.0
2: 00000000 0 FUNC LOCAL DEFAULT 2 __setup_mmu

Working (.type then label)

$ ./llvm-mc --triple=armv7a-linux-gnueabihf /tmp/test.s -filetype=obj -o /tmp/test.o --arm-add-build-attributes
$ ./llvm-readelf --symbols /tmp/test.o

Symbol table '.symtab' contains 3 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 00000000 0 NOTYPE LOCAL DEFAULT UND
1: 00000000 0 NOTYPE LOCAL DEFAULT 2 $t.0
2: 00000001 0 FUNC LOCAL DEFAULT 2 __setup_mmu << value = 1 here

I assume the "value" here is the issue?

@smithp35
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Yes, the type STT_FUNC says the state (ARM/Thumb) of the symbol is determined by the bottom bit. So when STT_FUNC is set bit0 == 0 means the linker treats the symbol as ARM, bit 0 == 1 means Thumb. So this in effect causes the linker to change state incorrectly causing an illegal instruction.

@agners
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agners commented Feb 20, 2020

Submitted a patch for review to fix this issue: https://reviews.llvm.org/D74927

@nickdesaulniers
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https://reviews.llvm.org/rGa921aaf789912d981cbb2036bdc91ad7289e1523

@llvmbot llvmbot transferred this issue from llvm/llvm-bugzilla-archive Dec 10, 2021
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@agners @DavidSpickett @nickdesaulniers @smithp35