6b1cf00400
Runtime code modification used by static keys is the most ubiquitous self-modifying feature of the Linux kernel. The idea is to to eliminate the condition check and associated conditional jump on a hot path if that condition (based on a boolean value of a static key) does not change often. Whenever they condition changes, the kernel runtime modifies all code paths associated with that key flipping the code between nop and (unconditional) jump.
68 lines
1.6 KiB
ArmAsm
68 lines
1.6 KiB
ArmAsm
# REQUIRES: system-linux
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## Check that BOLT correctly updates the Linux kernel static keys jump table.
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# RUN: llvm-mc -filetype=obj -triple x86_64-unknown-unknown %s -o %t.o
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# RUN: %clang %cflags -nostdlib %t.o -o %t.exe \
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# RUN: -Wl,--image-base=0xffffffff80000000,--no-dynamic-linker,--no-eh-frame-hdr
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## Verify static keys jump bindings to instructions.
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# RUN: llvm-bolt %t.exe --print-normalized -o %t.out --keep-nops=0 \
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# RUN: --bolt-info=0 |& FileCheck %s
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## Verify the bindings again on the rewritten binary with nops removed.
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# RUN: llvm-bolt %t.out -o %t.out.1 --print-normalized |& FileCheck %s
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# CHECK: BOLT-INFO: Linux kernel binary detected
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# CHECK: BOLT-INFO: parsed 2 static keys jump entries
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.text
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.globl _start
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.type _start, %function
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_start:
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# CHECK: Binary Function "_start"
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nop
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.L0:
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jmp .L1
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# CHECK: jit
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# CHECK-SAME: # ID: 1 {{.*}} # Likely: 0 # InitValue: 1
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nop
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.L1:
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.nops 5
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# CHECK: jit
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# CHECK-SAME: # ID: 2 {{.*}} # Likely: 1 # InitValue: 1
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.L2:
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nop
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.size _start, .-_start
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.globl foo
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.type foo, %function
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foo:
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ret
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.size foo, .-foo
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## Static keys jump table.
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.rodata
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.globl __start___jump_table
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.type __start___jump_table, %object
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__start___jump_table:
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.long .L0 - . # Jump address
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.long .L1 - . # Target address
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.quad 1 # Key address
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.long .L1 - . # Jump address
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.long .L2 - . # Target address
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.quad 0 # Key address
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.globl __stop___jump_table
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.type __stop___jump_table, %object
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__stop___jump_table:
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## Fake Linux Kernel sections.
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.section __ksymtab,"a",@progbits
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.section __ksymtab_gpl,"a",@progbits
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