[all-commits] [llvm/llvm-project] 5888de: [ARM, ELF] Fix access to dso_preemptable __stack_ch...

[Fangrui Song via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 5888dee7d04748744743a35d3aef030018bdc275
      https://github.com/llvm/llvm-project/commit/5888dee7d04748744743a35d3aef030018bdc275
  Author: Fangrui Song <i at maskray.me>
  Date:   2023-10-31 (Tue, 31 Oct 2023)

  Changed paths:
    M llvm/lib/Target/ARM/ARMBaseInstrInfo.cpp
    M llvm/lib/Target/ARM/ARMInstrInfo.cpp
    M llvm/test/CodeGen/ARM/stack-guard-elf.ll

  Log Message:
  -----------
  [ARM,ELF] Fix access to dso_preemptable __stack_chk_guard with static relocation model (#70014)

The ELF code from https://reviews.llvm.org/D112811 emits LDRLIT_ga_pcrel
when `TM.isPositionIndependent()` but uses a different condition
`Subtarget.isGVIndirectSymbol(GV)` (aka dso_preemptable on ELF targets).
This would cause incorrect access for dso_preemptable
`__stack_chk_guard` with the static relocation model.

Regarding whether `__stack_chk_guard` gets the dso_local specifier,
https://reviews.llvm.org/D150841 switched to
`M.getDirectAccessExternalData()` (implied by "PIC Level") instead of
`TM.getRelocationModel() == Reloc::Static`.

The result is that when non-zero "PIC Level" is used with static
relocation model (e.g. -fPIE/-fPIC LTO compiles with -no-pie linking),
`__stack_chk_guard` accesses are incorrect.
```
        ldr     r0, .LCPI0_0
        ldr     r0, [r0]
        ldr     r0, [r0]   // incorrectly dereferences __stack_chk_guard
...
.LCPI0_0:
        .long   __stack_chk_guard
```

To fix this, for dso_preemptable `__stack_chk_guard`, emit a GOT PIC
code sequence like for -fpic using `LDRLIT_ga_pcrel`:
```
        ldr     r0, .LCPI0_0
.LPC0_0:
        add     r0, pc, r0
        ldr     r0, [r0]
        ldr     r0, [r0]
...
LCPI0_0:
.Ltmp0:
        .long   __stack_chk_guard(GOT_PREL)-((.LPC0_0+8)-.Ltmp0)
```

Technically, `LDRLIT_ga_abs` with `R_ARM_GOT_ABS` could be used, but
`R_ARM_GOT_ABS` does not have GNU or integrated assembler support.
(Note, `.LCPI0_0: .long __stack_chk_guard at GOT` produces an
`R_ARM_GOT_BREL`, which is not desired).

This patch fixes #6499 while not changing behavior for the following
configurations:
```
run arm.linux.nopic --target=arm-linux-gnueabi -fno-pic
run arm.linux.pie --target=arm-linux-gnueabi -fpie
run arm.linux.pic --target=arm-linux-gnueabi -fpic
run armv6.darwin.nopic --target=armv6-apple-darwin -fno-pic
run armv6.darwin.dynamicnopic --target=armv6-apple-darwin -mdynamic-no-pic
run armv6.darwin.pic --target=armv6-apple-darwin -fpic
run armv7.darwin.nopic --target=armv7-apple-darwin -mcpu=cortex-a8 -fno-pic
run armv7.darwin.dynamicnopic --target=armv7-apple-darwin -mcpu=cortex-a8 -mdynamic-no-pic
run armv7.darwin.pic --target=armv7-apple-darwin -mcpu=cortex-a8 -fpic
run arm64.darwin.pic --target=arm64-apple-darwin
```