Re: [PATCH] riscv: Fix spurious errors from __get/put_kernel_nofault

[Charlie Jenkins <charlie@rivosinc.com>]

On Tue, Mar 12, 2024 at 10:05:37PM -0500, Samuel Holland wrote:
> On 2024-03-12 9:53 PM, Charlie Jenkins wrote:
> > 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.
> >>
> >> [1]: https://lore.kernel.org/linux-riscv/20240308-disable_misaligned_probe_config-v9-0-a388770ba0ce@rivosinc.com/
> >>
> >>  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
> >>
> >>
> >> _______________________________________________
> >> linux-riscv mailing list
> >> linux-riscv@lists.infradead.org
> >> http://lists.infradead.org/mailman/listinfo/linux-riscv
> > 
> > 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
>