On Tue, Mar 12, 2024 at 10:05:37PM -0500, Samuel Holland wrote:

On 2024-03-12 9:53 PM, Charlie Jenkins wrote:

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On Mon, Mar 11, 2024 at 07:19:13PM -0700, Samuel Holland wrote:

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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

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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.

extable works by running the handler (selected by EX_TYPE_*) if some exception occurs while executing that instruction -- see the calls to fixup_exception() in fault.c and traps.c. If there is no exception, then the handler does not run, and the err register is not written by ex_handler_uaccess_err_zero().

Hmm okay I understand thank you for explaining that. It's interesting to me that in __get_user_asm 'err' is set as a read/write variable even though __get_user_asm doesn't write to it. However, it seems like changing it to a write-only variable the compiler incorrectly optimizes err, and the kernel fails to boot.

If you look at __get_user_asm(), you can see that the err register is not touched by the assembly code at all -- the only reference to %0 is in the extable entry. So if the macro that declares the error variable doesn't initialize it, nothing will.

Reviewed-by: Charlie Jenkins charlie@rivosinc.com

Compare __get_user() and __put_user() which do initialize their error variable.

Regards, Samuel