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

[Ard Biesheuvel]

From: Ard Biesheuvel <ardb@xxxxxxxxxx>

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@xxxxxxxxxx>
Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Cc: Bill Wendling <morbo@xxxxxxxxxx>
Cc: Justin Stitt <justinstitt@xxxxxxxxxx>
Cc: Song Liu <song@xxxxxxxxxx>
Cc: Ricardo Ribalda <ribalda@xxxxxxxxxx>
Cc: Fangrui Song <maskray@xxxxxxxxxx>
Cc: Arthur Eubanks <aeubanks@xxxxxxxxxx>
Link: https://lore.kernel.org/all/20240417-x86-fix-kexec-with-llvm-18-v1-0-5383121e8fb7@xxxxxxxxxx/
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
---
 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