Re: [PATCH] x86/purgatory: Switch to the position-independent small code model

[Nathan Chancellor <nathan@kernel.org>]

On Thu, Apr 18, 2024 at 10:17:06PM +0200, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@kernel.org>
> 
> On x86, the ordinary, position dependent 'small' and 'kernel' code models only
> support placement of the executable in 32-bit addressable memory, due to
> the use of 32-bit signed immediates to generate references to global
> variables. For the kernel, this implies that all global variables must
> reside in the top 2 GiB of the kernel virtual address space, where the
> implicit address bits 63:32 are equal to sign bit 31.
> 
> This means the kernel code model is not suitable for other bare metal
> executables such as the kexec purgatory, which can be placed arbitrarily
> in the physical address space, where its address may no longer be
> representable as a sign extended 32-bit quantity. For this reason,
> commit
> 
>   e16c2983fba0 ("x86/purgatory: Change compiler flags from -mcmodel=kernel to -mcmodel=large to fix kexec relocation errors")
> 
> switched to the 'large' code model, which uses 64-bit immediates for all
> symbol references, including function calls, in order to avoid relying
> on any assumptions regarding proximity of symbols in the final
> executable.
> 
> The large code model is rarely used, clunky and the least likely to
> operate in a similar fashion when comparing GCC and Clang, so it is best
> avoided. This is especially true now that Clang 18 has started to emit
> executable code in two separate sections (.text and .ltext), which
> triggers an issue in the kexec loading code at runtime.
> 
> Instead, use the position independent small code model, which makes no
> assumptions about placement but only about proximity, where all
> referenced symbols must be within -/+ 2 GiB, i.e., in range for a
> RIP-relative reference. Use hidden visibility to suppress the use of a
> GOT, which carries absolute addresses that are not covered by static ELF
> relocations, and is therefore incompatible with the kexec loader's
> relocation logic.
> 
> Cc: Nathan Chancellor <nathan@kernel.org>
> Cc: Nick Desaulniers <ndesaulniers@google.com>
> Cc: Bill Wendling <morbo@google.com>
> Cc: Justin Stitt <justinstitt@google.com>
> Cc: Song Liu <song@kernel.org>
> Cc: Ricardo Ribalda <ribalda@kernel.org>
> Cc: Fangrui Song <maskray@google.com>
> Cc: Arthur Eubanks <aeubanks@google.com>
> Link: https://lore.kernel.org/all/20240417-x86-fix-kexec-with-llvm-18-v1-0-5383121e8fb7@kernel.org/
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>

This resolves the warning and relocation overflow error that I see with
LLVM 18.1.4 (the relocation error was fixed in LLVM but dropping
-mcmodel=large resolves it for toolchains without that fix).

> ---
>  arch/x86/purgatory/Makefile | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/purgatory/Makefile b/arch/x86/purgatory/Makefile
> index bc31863c5ee6..a18591f6e6d9 100644
> --- a/arch/x86/purgatory/Makefile
> +++ b/arch/x86/purgatory/Makefile
> @@ -42,7 +42,8 @@ KCOV_INSTRUMENT := n
>  # make up the standalone purgatory.ro
>  
>  PURGATORY_CFLAGS_REMOVE := -mcmodel=kernel
> -PURGATORY_CFLAGS := -mcmodel=large -ffreestanding -fno-zero-initialized-in-bss -g0
> +PURGATORY_CFLAGS := -mcmodel=small -ffreestanding -fno-zero-initialized-in-bss -g0
> +PURGATORY_CFLAGS += -fpic -fvisibility=hidden
>  PURGATORY_CFLAGS += $(DISABLE_STACKLEAK_PLUGIN) -DDISABLE_BRANCH_PROFILING
>  PURGATORY_CFLAGS += -fno-stack-protector
>  
> -- 
> 2.44.0.769.g3c40516874-goog
>