On Mon, Mar 11, 2024 at 07:19:13PM -0700, Samuel Holland wrote:
These macros did not initialize __kr_err, so they could fail even if the access did not fault.
Cc: stable@vger.kernel.org Fixes: d464118cdc41 ("riscv: implement __get_kernel_nofault and __put_user_nofault") Signed-off-by: Samuel Holland samuel.holland@sifive.com
Found while testing the unaligned access speed series[1]. The observed behavior was that with RISCV_EFFICIENT_UNALIGNED_ACCESS=y, the copy_from_kernel_nofault() in prepend_copy() failed every time when filling out /proc/self/mounts, so all of the mount points were "xxx".
I'm surprised this hasn't been seen before. For reference, I'm compiling with clang 18.
arch/riscv/include/asm/uaccess.h | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/arch/riscv/include/asm/uaccess.h b/arch/riscv/include/asm/uaccess.h index ec0cab9fbddd..72ec1d9bd3f3 100644 --- a/arch/riscv/include/asm/uaccess.h +++ b/arch/riscv/include/asm/uaccess.h @@ -319,7 +319,7 @@ unsigned long __must_check clear_user(void __user *to, unsigned long n) #define __get_kernel_nofault(dst, src, type, err_label) \ do { \
- long __kr_err; \
- long __kr_err = 0; \ \ __get_user_nocheck(*((type *)(dst)), (type *)(src), __kr_err); \ if (unlikely(__kr_err)) \
@@ -328,7 +328,7 @@ do { \ #define __put_kernel_nofault(dst, src, type, err_label) \ do { \
- long __kr_err; \
- long __kr_err = 0; \ \ __put_user_nocheck(*((type *)(src)), (type *)(dst), __kr_err); \ if (unlikely(__kr_err)) \
-- 2.43.1
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I am not able to reproduce this using Clang 18 with RISCV_EFFICIENT_UNALIGNED_ACCESS=y on 6.8. However I can see how this could be an issue.
Going down the rabbit hold of macros here, I end up at arch/riscv/include/asm/asm-extable.h where the register that hold 'err' is written into the __ex_table section:
#define EX_DATA_REG(reg, gpr) \ "((.L__gpr_num_" #gpr ") << " __stringify(EX_DATA_REG_##reg##_SHIFT) ")"
#define _ASM_EXTABLE_UACCESS_ERR_ZERO(insn, fixup, err, zero) \ __DEFINE_ASM_GPR_NUMS \ __ASM_EXTABLE_RAW(#insn, #fixup, \ __stringify(EX_TYPE_UACCESS_ERR_ZERO), \ "(" \ EX_DATA_REG(ERR, err) " | " \ EX_DATA_REG(ZERO, zero) \ ")")
I am wondering if setting this value to zero solves the problem by hiding another issue. It seems like this shouldn't need to be initialized to zero, however I am lost as to how this extable setup works so perhaps this is the proper solution.
- Charlie