On Thu, Apr 18, 2024 at 1:17 PM Ard Biesheuvel <ardb+git@xxxxxxxxxx> wrote: > > 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> Thanks Ard! Acked-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> Reported-by: ns <0n-s@xxxxxxxxxxxxxxxxxxxxxxxx> Closes: https://github.com/ClangBuiltLinux/linux/issues/2016 Fixes: e16c2983fba0 ("x86/purgatory: Change compiler flags from -mcmodel=kernel to -mcmodel=large to fix kexec relocation errors") (I don't have a kexec setup ready to go; maybe someone that does can help test it.) > 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 > -- Thanks, ~Nick Desaulniers