[PATCH v2 0/3] riscv: misaligned: fix misaligned accesses handling in put/get_user()

[PATCH v2 0/3] riscv: misaligned: fix misaligned accesses handling in put/get_user()

[Clément Léger]

While debugging a few problems with the misaligned access kselftest,
Alexandre discovered some crash with the current code. Indeed, some
misaligned access was done by the kernel using put_user(). This
was resulting in trap and a kernel crash since. The path was the
following:
user -> kernel -> access to user memory -> misaligned trap -> trap ->
kernel -> misaligned handling -> memcpy -> crash due to failed page fault
while in interrupt disabled section.

Last discussion about kernel misaligned handling and interrupt reenabling
were actually not to reenable interrupt when handling misaligned access
being done by kernel. The best solution being not to do any misaligned
accesses to userspace memory, we considered a few options:

- Remove any call to put/get_user() potentially doing misaligned
  accesses
- Do not do any misaligned accesses in put/get_user() itself

The second solution was the one chosen as there are too many callsites to
put/get_user() that could potentially do misaligned accesses. We tried
two approaches for that, either split access in two aligned accesses
(and do RMW for put_user()) or call copy_from/to_user() which does not
do any misaligned accesses. The later one was the simpler to implement
(although the performances are probably lower than split aligned
accesses but still way better than doing misaligned access emulation)
and allows to support what we wanted.

These commits are based on top of Alex dev/alex/get_user_misaligned_v1
branch.

---
v2:
 - Use __inttype instead of unsigned long for pointer cast
 - Add Alex patch to make unsafe func use existing assembly.
   - Add missing EXPORT_SYMBOL(__asm_copy_to_user_sum_enabled)

Alexandre Ghiti (1):
  riscv: make unsafe user copy routines use existing assembly routines

Clément Léger (2):
  riscv: process: use unsigned int instead of unsigned long for
    put_user()
  riscv: uaccess: do not do misaligned accesses in get/put_user()

 arch/riscv/include/asm/asm-prototypes.h |  2 +-
 arch/riscv/include/asm/uaccess.h        | 48 +++++++++++-------------
 arch/riscv/kernel/process.c             |  2 +-
 arch/riscv/lib/riscv_v_helpers.c        | 11 ++++--
 arch/riscv/lib/uaccess.S                | 50 +++++++++++++++++--------
 arch/riscv/lib/uaccess_vector.S         | 15 ++++++--
 6 files changed, 78 insertions(+), 50 deletions(-)

-- 
2.49.0


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[PATCH v2 1/3] riscv: make unsafe user copy routines use existing assembly routines

[Clément Léger]

From: Alexandre Ghiti <alexghiti@rivosinc.com>

The current implementation is underperforming and in addition, it
triggers misaligned access traps on platforms which do not handle
misaligned accesses in hardware.

Use the existing assembly routines to solve both problems at once.

Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
---
 arch/riscv/include/asm/asm-prototypes.h |  2 +-
 arch/riscv/include/asm/uaccess.h        | 33 ++++------------
 arch/riscv/lib/riscv_v_helpers.c        | 11 ++++--
 arch/riscv/lib/uaccess.S                | 50 +++++++++++++++++--------
 arch/riscv/lib/uaccess_vector.S         | 15 ++++++--
 5 files changed, 63 insertions(+), 48 deletions(-)

diff --git a/arch/riscv/include/asm/asm-prototypes.h b/arch/riscv/include/asm/asm-prototypes.h
index cd627ec289f1..5d10edde6d17 100644
--- a/arch/riscv/include/asm/asm-prototypes.h
+++ b/arch/riscv/include/asm/asm-prototypes.h
@@ -12,7 +12,7 @@ long long __ashlti3(long long a, int b);
 #ifdef CONFIG_RISCV_ISA_V
 
 #ifdef CONFIG_MMU
-asmlinkage int enter_vector_usercopy(void *dst, void *src, size_t n);
+asmlinkage int enter_vector_usercopy(void *dst, void *src, size_t n, bool enable_sum);
 #endif /* CONFIG_MMU  */
 
 void xor_regs_2_(unsigned long bytes, unsigned long *__restrict p1,
diff --git a/arch/riscv/include/asm/uaccess.h b/arch/riscv/include/asm/uaccess.h
index 87d01168f80a..046de7ced09c 100644
--- a/arch/riscv/include/asm/uaccess.h
+++ b/arch/riscv/include/asm/uaccess.h
@@ -450,35 +450,18 @@ static inline void user_access_restore(unsigned long enabled) { }
 	(x) = (__force __typeof__(*(ptr)))__gu_val;			\
 } while (0)
 
-#define unsafe_copy_loop(dst, src, len, type, op, label)		\
-	while (len >= sizeof(type)) {					\
-		op(*(type *)(src), (type __user *)(dst), label);	\
-		dst += sizeof(type);					\
-		src += sizeof(type);					\
-		len -= sizeof(type);					\
-	}
+unsigned long __must_check __asm_copy_to_user_sum_enabled(void __user *to,
+	const void *from, unsigned long n);
+unsigned long __must_check __asm_copy_from_user_sum_enabled(void *to,
+	const void __user *from, unsigned long n);
 
 #define unsafe_copy_to_user(_dst, _src, _len, label)			\
-do {									\
-	char __user *__ucu_dst = (_dst);				\
-	const char *__ucu_src = (_src);					\
-	size_t __ucu_len = (_len);					\
-	unsafe_copy_loop(__ucu_dst, __ucu_src, __ucu_len, u64, unsafe_put_user, label);	\
-	unsafe_copy_loop(__ucu_dst, __ucu_src, __ucu_len, u32, unsafe_put_user, label);	\
-	unsafe_copy_loop(__ucu_dst, __ucu_src, __ucu_len, u16, unsafe_put_user, label);	\
-	unsafe_copy_loop(__ucu_dst, __ucu_src, __ucu_len, u8, unsafe_put_user, label);	\
-} while (0)
+	if (__asm_copy_to_user_sum_enabled(_dst, _src, _len))		\
+		goto label;
 
 #define unsafe_copy_from_user(_dst, _src, _len, label)			\
-do {									\
-	char *__ucu_dst = (_dst);					\
-	const char __user *__ucu_src = (_src);				\
-	size_t __ucu_len = (_len);					\
-	unsafe_copy_loop(__ucu_src, __ucu_dst, __ucu_len, u64, unsafe_get_user, label);	\
-	unsafe_copy_loop(__ucu_src, __ucu_dst, __ucu_len, u32, unsafe_get_user, label);	\
-	unsafe_copy_loop(__ucu_src, __ucu_dst, __ucu_len, u16, unsafe_get_user, label);	\
-	unsafe_copy_loop(__ucu_src, __ucu_dst, __ucu_len, u8, unsafe_get_user, label);	\
-} while (0)
+	if (__asm_copy_from_user_sum_enabled(_dst, _src, _len))		\
+		goto label;
 
 #else /* CONFIG_MMU */
 #include <asm-generic/uaccess.h>
diff --git a/arch/riscv/lib/riscv_v_helpers.c b/arch/riscv/lib/riscv_v_helpers.c
index be38a93cedae..7bbdfc6d4552 100644
--- a/arch/riscv/lib/riscv_v_helpers.c
+++ b/arch/riscv/lib/riscv_v_helpers.c
@@ -16,8 +16,11 @@
 #ifdef CONFIG_MMU
 size_t riscv_v_usercopy_threshold = CONFIG_RISCV_ISA_V_UCOPY_THRESHOLD;
 int __asm_vector_usercopy(void *dst, void *src, size_t n);
+int __asm_vector_usercopy_sum_enabled(void *dst, void *src, size_t n);
 int fallback_scalar_usercopy(void *dst, void *src, size_t n);
-asmlinkage int enter_vector_usercopy(void *dst, void *src, size_t n)
+int fallback_scalar_usercopy_sum_enabled(void *dst, void *src, size_t n);
+asmlinkage int enter_vector_usercopy(void *dst, void *src, size_t n,
+				     bool enable_sum)
 {
 	size_t remain, copied;
 
@@ -26,7 +29,8 @@ asmlinkage int enter_vector_usercopy(void *dst, void *src, size_t n)
 		goto fallback;
 
 	kernel_vector_begin();
-	remain = __asm_vector_usercopy(dst, src, n);
+	remain = enable_sum ? __asm_vector_usercopy(dst, src, n) :
+			      __asm_vector_usercopy_sum_enabled(dst, src, n);
 	kernel_vector_end();
 
 	if (remain) {
@@ -40,6 +44,7 @@ asmlinkage int enter_vector_usercopy(void *dst, void *src, size_t n)
 	return remain;
 
 fallback:
-	return fallback_scalar_usercopy(dst, src, n);
+	return enable_sum ? fallback_scalar_usercopy(dst, src, n) :
+			    fallback_scalar_usercopy_sum_enabled(dst, src, n);
 }
 #endif
diff --git a/arch/riscv/lib/uaccess.S b/arch/riscv/lib/uaccess.S
index 6a9f116bb545..4efea1b3326c 100644
--- a/arch/riscv/lib/uaccess.S
+++ b/arch/riscv/lib/uaccess.S
@@ -17,14 +17,43 @@ SYM_FUNC_START(__asm_copy_to_user)
 	ALTERNATIVE("j fallback_scalar_usercopy", "nop", 0, RISCV_ISA_EXT_ZVE32X, CONFIG_RISCV_ISA_V)
 	REG_L	t0, riscv_v_usercopy_threshold
 	bltu	a2, t0, fallback_scalar_usercopy
-	tail enter_vector_usercopy
+	li	a3, 1
+	tail 	enter_vector_usercopy
 #endif
-SYM_FUNC_START(fallback_scalar_usercopy)
+SYM_FUNC_END(__asm_copy_to_user)
+EXPORT_SYMBOL(__asm_copy_to_user)
+SYM_FUNC_ALIAS(__asm_copy_from_user, __asm_copy_to_user)
+EXPORT_SYMBOL(__asm_copy_from_user)
 
+SYM_FUNC_START(fallback_scalar_usercopy)
 	/* Enable access to user memory */
-	li t6, SR_SUM
-	csrs CSR_STATUS, t6
+	li	t6, SR_SUM
+	csrs 	CSR_STATUS, t6
+	mv 	t6, ra
 
+	call 	fallback_scalar_usercopy_sum_enabled
+
+	/* Disable access to user memory */
+	mv 	ra, t6
+	li 	t6, SR_SUM
+	csrc 	CSR_STATUS, t6
+	ret
+SYM_FUNC_END(fallback_scalar_usercopy)
+
+SYM_FUNC_START(__asm_copy_to_user_sum_enabled)
+#ifdef CONFIG_RISCV_ISA_V
+	ALTERNATIVE("j fallback_scalar_usercopy_sum_enabled", "nop", 0, RISCV_ISA_EXT_ZVE32X, CONFIG_RISCV_ISA_V)
+	REG_L	t0, riscv_v_usercopy_threshold
+	bltu	a2, t0, fallback_scalar_usercopy_sum_enabled
+	li	a3, 0
+	tail 	enter_vector_usercopy
+#endif
+SYM_FUNC_END(__asm_copy_to_user_sum_enabled)
+SYM_FUNC_ALIAS(__asm_copy_from_user_sum_enabled, __asm_copy_to_user_sum_enabled)
+EXPORT_SYMBOL(__asm_copy_from_user_sum_enabled)
+EXPORT_SYMBOL(__asm_copy_to_user_sum_enabled)
+
+SYM_FUNC_START(fallback_scalar_usercopy_sum_enabled)
 	/*
 	 * Save the terminal address which will be used to compute the number
 	 * of bytes copied in case of a fixup exception.
@@ -178,23 +207,12 @@ SYM_FUNC_START(fallback_scalar_usercopy)
 	bltu	a0, t0, 4b	/* t0 - end of dst */
 
 .Lout_copy_user:
-	/* Disable access to user memory */
-	csrc CSR_STATUS, t6
 	li	a0, 0
 	ret
-
-	/* Exception fixup code */
 10:
-	/* Disable access to user memory */
-	csrc CSR_STATUS, t6
 	sub a0, t5, a0
 	ret
-SYM_FUNC_END(__asm_copy_to_user)
-SYM_FUNC_END(fallback_scalar_usercopy)
-EXPORT_SYMBOL(__asm_copy_to_user)
-SYM_FUNC_ALIAS(__asm_copy_from_user, __asm_copy_to_user)
-EXPORT_SYMBOL(__asm_copy_from_user)
-
+SYM_FUNC_END(fallback_scalar_usercopy_sum_enabled)
 
 SYM_FUNC_START(__clear_user)
 
diff --git a/arch/riscv/lib/uaccess_vector.S b/arch/riscv/lib/uaccess_vector.S
index 7c45f26de4f7..03b5560609a2 100644
--- a/arch/riscv/lib/uaccess_vector.S
+++ b/arch/riscv/lib/uaccess_vector.S
@@ -24,7 +24,18 @@ SYM_FUNC_START(__asm_vector_usercopy)
 	/* Enable access to user memory */
 	li	t6, SR_SUM
 	csrs	CSR_STATUS, t6
+	mv	t6, ra
 
+	call 	__asm_vector_usercopy_sum_enabled
+
+	/* Disable access to user memory */
+	mv 	ra, t6
+	li 	t6, SR_SUM
+	csrc	CSR_STATUS, t6
+	ret
+SYM_FUNC_END(__asm_vector_usercopy)
+
+SYM_FUNC_START(__asm_vector_usercopy_sum_enabled)
 loop:
 	vsetvli iVL, iNum, e8, ELEM_LMUL_SETTING, ta, ma
 	fixup vle8.v vData, (pSrc), 10f
@@ -36,8 +47,6 @@ loop:
 
 	/* Exception fixup for vector load is shared with normal exit */
 10:
-	/* Disable access to user memory */
-	csrc	CSR_STATUS, t6
 	mv	a0, iNum
 	ret
 
@@ -49,4 +58,4 @@ loop:
 	csrr	t2, CSR_VSTART
 	sub	iNum, iNum, t2
 	j	10b
-SYM_FUNC_END(__asm_vector_usercopy)
+SYM_FUNC_END(__asm_vector_usercopy_sum_enabled)
-- 
2.49.0


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[PATCH v2 2/3] riscv: process: use unsigned int instead of unsigned long for put_user()

[Clément Léger]

The specification of prctl() for GET_UNALIGN_CTL states that the value is
returned in an unsigned int * address passed as an unsigned long. Change
the type to match that and avoid an unaligned access as well.

Signed-off-by: Clément Léger <cleger@rivosinc.com>
---
 arch/riscv/kernel/process.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
index 15d8f75902f8..9ee6d816b98b 100644
--- a/arch/riscv/kernel/process.c
+++ b/arch/riscv/kernel/process.c
@@ -57,7 +57,7 @@ int get_unalign_ctl(struct task_struct *tsk, unsigned long adr)
 	if (!unaligned_ctl_available())
 		return -EINVAL;
 
-	return put_user(tsk->thread.align_ctl, (unsigned long __user *)adr);
+	return put_user(tsk->thread.align_ctl, (unsigned int __user *)adr);
 }
 
 void __show_regs(struct pt_regs *regs)
-- 
2.49.0


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Re: [PATCH v2 2/3] riscv: process: use unsigned int instead of unsigned long for put_user()

[Alexandre Ghiti]

Hi Clément,

On 6/2/25 21:39, Clément Léger wrote:
> The specification of prctl() for GET_UNALIGN_CTL states that the value is
> returned in an unsigned int * address passed as an unsigned long. Change
> the type to match that and avoid an unaligned access as well.
>
> Signed-off-by: Clément Léger <cleger@rivosinc.com>
> ---
>   arch/riscv/kernel/process.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
> index 15d8f75902f8..9ee6d816b98b 100644
> --- a/arch/riscv/kernel/process.c
> +++ b/arch/riscv/kernel/process.c
> @@ -57,7 +57,7 @@ int get_unalign_ctl(struct task_struct *tsk, unsigned long adr)
>   	if (!unaligned_ctl_available())
>   		return -EINVAL;
>   
> -	return put_user(tsk->thread.align_ctl, (unsigned long __user *)adr);
> +	return put_user(tsk->thread.align_ctl, (unsigned int __user *)adr);
>   }
>   
>   void __show_regs(struct pt_regs *regs)


Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>

Thanks,

Alex


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[PATCH v2 3/3] riscv: uaccess: do not do misaligned accesses in get/put_user()

[Clément Léger]

Doing misaligned access to userspace memory would make a trap on
platform where it is emulated. Latest fixes removed the kernel
capability to do unaligned accesses to userspace memory safely since
interrupts are kept disabled at all time during that. Thus doing so
would crash the kernel.

Such behavior was detected with GET_UNALIGN_CTL() that was doing
a put_user() with an unsigned long* address that should have been an
unsigned int*. Reenabling kernel misaligned access emulation is a bit
risky and it would also degrade performances. Rather than doing that,
we will try to avoid any misaligned accessed by using copy_from/to_user()
which does not do any misaligned accesses. This can be done only for
!CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS and thus allows to only generate
a bit more code for this config.

Signed-off-by: Clément Léger <cleger@rivosinc.com>
---
 arch/riscv/include/asm/uaccess.h | 23 ++++++++++++++++++-----
 1 file changed, 18 insertions(+), 5 deletions(-)

diff --git a/arch/riscv/include/asm/uaccess.h b/arch/riscv/include/asm/uaccess.h
index 046de7ced09c..d472da4450e6 100644
--- a/arch/riscv/include/asm/uaccess.h
+++ b/arch/riscv/include/asm/uaccess.h
@@ -169,8 +169,19 @@ do {								\
 
 #endif /* CONFIG_64BIT */
 
+unsigned long __must_check __asm_copy_to_user_sum_enabled(void __user *to,
+	const void *from, unsigned long n);
+unsigned long __must_check __asm_copy_from_user_sum_enabled(void *to,
+	const void __user *from, unsigned long n);
+
 #define __get_user_nocheck(x, __gu_ptr, label)			\
 do {								\
+	if (!IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) &&	\
+	    !IS_ALIGNED((uintptr_t)__gu_ptr, sizeof(*__gu_ptr))) {	\
+		if (__asm_copy_from_user_sum_enabled(&(x), __gu_ptr, sizeof(*__gu_ptr))) \
+			goto label;				\
+		break;						\
+	}							\
 	switch (sizeof(*__gu_ptr)) {				\
 	case 1:							\
 		__get_user_asm("lb", (x), __gu_ptr, label);	\
@@ -297,6 +308,13 @@ do {								\
 
 #define __put_user_nocheck(x, __gu_ptr, label)			\
 do {								\
+	if (!IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) &&	\
+	    !IS_ALIGNED((uintptr_t)__gu_ptr, sizeof(*__gu_ptr))) {	\
+		__inttype(x) val = (__inttype(x))x;			\
+		if (__asm_copy_to_user_sum_enabled(__gu_ptr, &(val), sizeof(*__gu_ptr))) \
+			goto label;				\
+		break;						\
+	}							\
 	switch (sizeof(*__gu_ptr)) {				\
 	case 1:							\
 		__put_user_asm("sb", (x), __gu_ptr, label);	\
@@ -450,11 +468,6 @@ static inline void user_access_restore(unsigned long enabled) { }
 	(x) = (__force __typeof__(*(ptr)))__gu_val;			\
 } while (0)
 
-unsigned long __must_check __asm_copy_to_user_sum_enabled(void __user *to,
-	const void *from, unsigned long n);
-unsigned long __must_check __asm_copy_from_user_sum_enabled(void *to,
-	const void __user *from, unsigned long n);
-
 #define unsafe_copy_to_user(_dst, _src, _len, label)			\
 	if (__asm_copy_to_user_sum_enabled(_dst, _src, _len))		\
 		goto label;
-- 
2.49.0


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Re: [PATCH v2 3/3] riscv: uaccess: do not do misaligned accesses in get/put_user()

[Alexandre Ghiti]

On 6/2/25 21:39, Clément Léger wrote:
> Doing misaligned access to userspace memory would make a trap on
> platform where it is emulated. Latest fixes removed the kernel
> capability to do unaligned accesses to userspace memory safely since
> interrupts are kept disabled at all time during that. Thus doing so
> would crash the kernel.
>
> Such behavior was detected with GET_UNALIGN_CTL() that was doing
> a put_user() with an unsigned long* address that should have been an
> unsigned int*. Reenabling kernel misaligned access emulation is a bit
> risky and it would also degrade performances. Rather than doing that,
> we will try to avoid any misaligned accessed by using copy_from/to_user()
> which does not do any misaligned accesses. This can be done only for
> !CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS and thus allows to only generate
> a bit more code for this config.
>
> Signed-off-by: Clément Léger <cleger@rivosinc.com>
> ---
>   arch/riscv/include/asm/uaccess.h | 23 ++++++++++++++++++-----
>   1 file changed, 18 insertions(+), 5 deletions(-)
>
> diff --git a/arch/riscv/include/asm/uaccess.h b/arch/riscv/include/asm/uaccess.h
> index 046de7ced09c..d472da4450e6 100644
> --- a/arch/riscv/include/asm/uaccess.h
> +++ b/arch/riscv/include/asm/uaccess.h
> @@ -169,8 +169,19 @@ do {								\
>   
>   #endif /* CONFIG_64BIT */
>   
> +unsigned long __must_check __asm_copy_to_user_sum_enabled(void __user *to,
> +	const void *from, unsigned long n);
> +unsigned long __must_check __asm_copy_from_user_sum_enabled(void *to,
> +	const void __user *from, unsigned long n);
> +
>   #define __get_user_nocheck(x, __gu_ptr, label)			\
>   do {								\
> +	if (!IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) &&	\
> +	    !IS_ALIGNED((uintptr_t)__gu_ptr, sizeof(*__gu_ptr))) {	\
> +		if (__asm_copy_from_user_sum_enabled(&(x), __gu_ptr, sizeof(*__gu_ptr))) \
> +			goto label;				\
> +		break;						\
> +	}							\
>   	switch (sizeof(*__gu_ptr)) {				\
>   	case 1:							\
>   		__get_user_asm("lb", (x), __gu_ptr, label);	\
> @@ -297,6 +308,13 @@ do {								\
>   
>   #define __put_user_nocheck(x, __gu_ptr, label)			\
>   do {								\
> +	if (!IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) &&	\
> +	    !IS_ALIGNED((uintptr_t)__gu_ptr, sizeof(*__gu_ptr))) {	\
> +		__inttype(x) val = (__inttype(x))x;			\
> +		if (__asm_copy_to_user_sum_enabled(__gu_ptr, &(val), sizeof(*__gu_ptr))) \
> +			goto label;				\
> +		break;						\
> +	}							\
>   	switch (sizeof(*__gu_ptr)) {				\
>   	case 1:							\
>   		__put_user_asm("sb", (x), __gu_ptr, label);	\
> @@ -450,11 +468,6 @@ static inline void user_access_restore(unsigned long enabled) { }
>   	(x) = (__force __typeof__(*(ptr)))__gu_val;			\
>   } while (0)
>   
> -unsigned long __must_check __asm_copy_to_user_sum_enabled(void __user *to,
> -	const void *from, unsigned long n);
> -unsigned long __must_check __asm_copy_from_user_sum_enabled(void *to,
> -	const void __user *from, unsigned long n);
> -
>   #define unsafe_copy_to_user(_dst, _src, _len, label)			\
>   	if (__asm_copy_to_user_sum_enabled(_dst, _src, _len))		\
>   		goto label;


Even if as noted by David, there is room for improvement, for now the 
simplicity of this is good enough to me, so:

Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>

Thanks,

Alex


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Re: [PATCH v2 0/3] riscv: misaligned: fix misaligned accesses handling in put/get_user()

[David Laight]

On Mon,  2 Jun 2025 21:39:13 +0200
Clément Léger <cleger@rivosinc.com> wrote:

...
> The second solution was the one chosen as there are too many callsites to
> put/get_user() that could potentially do misaligned accesses. We tried
> two approaches for that, either split access in two aligned accesses
> (and do RMW for put_user())

You can't do RMW because it is visible to other threads.

	David

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Re: [PATCH v2 0/3] riscv: misaligned: fix misaligned accesses handling in put/get_user()

[Clément Léger]

On 02/06/2025 23:08, David Laight wrote:
> On Mon,  2 Jun 2025 21:39:13 +0200
> Clément Léger <cleger@rivosinc.com> wrote:
> 
> ...
>> The second solution was the one chosen as there are too many callsites to
>> put/get_user() that could potentially do misaligned accesses. We tried
>> two approaches for that, either split access in two aligned accesses
>> (and do RMW for put_user())
> 
> You can't do RMW because it is visible to other threads.

Ahem yeah indeed, so the only way was to do individual lb at least up to
some alignment.

Thanks,

Clément

> 
> 	David


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Re: [PATCH v2 0/3] riscv: misaligned: fix misaligned accesses handling in put/get_user()

[patchwork-bot+linux-riscv]

Hello:

This series was applied to riscv/linux.git (for-next)
by Palmer Dabbelt <palmer@dabbelt.com>:

On Mon,  2 Jun 2025 21:39:13 +0200 you wrote:
> While debugging a few problems with the misaligned access kselftest,
> Alexandre discovered some crash with the current code. Indeed, some
> misaligned access was done by the kernel using put_user(). This
> was resulting in trap and a kernel crash since. The path was the
> following:
> user -> kernel -> access to user memory -> misaligned trap -> trap ->
> kernel -> misaligned handling -> memcpy -> crash due to failed page fault
> while in interrupt disabled section.
> 
> [...]

Here is the summary with links:
  - [v2,1/3] riscv: make unsafe user copy routines use existing assembly routines
    https://git.kernel.org/riscv/c/a4348546332c
  - [v2,2/3] riscv: process: use unsigned int instead of unsigned long for put_user()
    https://git.kernel.org/riscv/c/020667d661f9
  - [v2,3/3] riscv: uaccess: do not do misaligned accesses in get/put_user()
    https://git.kernel.org/riscv/c/ca1a66cdd685

You are awesome, thank you!
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