Re: [PATCH v1] xen/riscv: make calculation of stack address PC-relative

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.03.2023 21:16, Oleksii wrote:
> I checked in Linux binary how 'la' instruction is transformed, and it
> looks like it is translated as I expect to auipc/addi pair:
> ffffffe000001066: 00027517 auipc a0,0x27
> ffffffe00000106a: f9a50513 addi a0,a0,-102 # ffffffe000028000
> <early_pg_dir>
> 
> I checked compiler flags between Xen and Linux. The difference is in-
> fno-PIE (Linux also adds -mabi and -march to AFLAGS):
> 
> 1. Linux build command of head.S: riscv64-linux-gnu-gcc -Wp,-
> MD,arch/riscv/kernel/.head.o.d -nostdinc -isystem /usr/lib/gcc-
> cross/riscv64-linux-gnu/9/include -I./arch/riscv/include -
> I./arch/riscv/include/generated -I./include -I./arch/riscv/include/uapi
> -I./arch/riscv/include/generated/uapi -I./include/uapi -
> I./include/generated/uapi -include ./include/linux/kconfig.h -
> D__KERNEL__ -D__ASSEMBLY__ -fno-PIE -mabi=lp64 -march=rv64imafdc -c -o
> arch/riscv/kernel/head.o arch/riscv/kernel/head.S
> 
> 2. Xen build command of head.S:riscv64-linux-gnu-gcc -MMD -MP -MF
> arch/riscv/riscv64/.head.o.d -D__ASSEMBLY__ -Wa,--noexecstack -
> DBUILD_ID -fno-strict-aliasing -Wall -Wstrict-prototypes -Wdeclaration-
> after-statement -Wno-unused-but-set-variable -Wno-unused-local-typedefs
> -O1 -fno-omit-frame-pointer -nostdinc -fno-builtin -fno-common -Werror
> -Wredundant-decls -Wno-pointer-arith -Wvla -pipe -D__XEN__ -include
> ./include/xen/config.h -Wa,--strip-local-absolute -g -mabi=lp64 -
> I./include -I./arch/riscv/include -march=rv64gc -mstrict-align -
> mcmodel=medany - -c arch/riscv/riscv64/head.S -o
> arch/riscv/riscv64/head.o

Looking into why you see different code generated than I: Nothing in
here directs gcc to pass -fpic to gas; in upstream gcc (consistent
from gcc7 through gcc12, which are the versions I've checked; the
actual range may be wider) there is

#define ASM_SPEC "\
%(subtarget_asm_debugging_spec) \
%{" FPIE_OR_FPIC_SPEC ":-fpic} \
...

Can you check whether your gcc passes -fpic to gas even when there's
no -fPIC / -fPIE (or alike) on the gcc command line? Or whether your
gas (unlike upstream's) defaults to PIC mode? (For .S files ASM_SPEC
is all that counts. For .c files gcc is redundantly passing -fpic
along with also emitting ".option pic" or, in the opposite case, it
is omitting -fpic along with emitting ".option nopic".)

You gcc may have been configured with --enable-default-pie, while I
know mine hasn't been (simply because that's the default).

Jan