Re: [PATCH] powerpc: Don't corrupt user registers on 32-bit

[shiva7 <sivasakthi7@outlook.com>]

Paul Mackerras wrote
> Commit de79f7b9f6 ("powerpc: Put FP/VSX and VR state into structures")
> modified load_up_fpu() and load_up_altivec() in such a way that they
> now use r7 and r8.  Unfortunately, the callers of these functions on
> 32-bit machines then return to userspace via fast_exception_return,
> which doesn't restore all of the volatile GPRs, but only r1, r3 -- r6
> and r9 -- r12.  This was causing userspace segfaults and other
> userspace misbehaviour on 32-bit machines.
> 
> This fixes the problem by changing the register usage of load_up_fpu()
> and load_up_altivec() to avoid using r7 and r8 and instead use r6 and
> r10.  This also adds comments to those functions saying which registers
> may be used.
> 
> Signed-off-by: Paul Mackerras <

> paulus@

> >
> ---
>  arch/powerpc/kernel/fpu.S    | 14 ++++++++------
>  arch/powerpc/kernel/vector.S | 15 +++++++++------
>  2 files changed, 17 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/fpu.S b/arch/powerpc/kernel/fpu.S
> index 4dca05e..f7f5b8b 100644
> --- a/arch/powerpc/kernel/fpu.S
> +++ b/arch/powerpc/kernel/fpu.S
> @@ -106,6 +106,8 @@ _GLOBAL(store_fp_state)
>   * and save its floating-point registers in its thread_struct.
>   * Load up this task's FP registers from its thread_struct,
>   * enable the FPU for the current task and return to the task.
> + * Note that on 32-bit this can only use registers that will be
> + * restored by fast_exception_return, i.e. r3 - r6, r10 and r11.
>   */
>  _GLOBAL(load_up_fpu)
>  	mfmsr	r5
> @@ -131,10 +133,10 @@ END_FTR_SECTION_IFSET(CPU_FTR_VSX)
>  	beq	1f
>  	toreal(r4)
>  	addi	r4,r4,THREAD		/* want last_task_used_math->thread */
> -	addi	r8,r4,THREAD_FPSTATE
> -	SAVE_32FPVSRS(0, R5, R8)
> +	addi	r10,r4,THREAD_FPSTATE
> +	SAVE_32FPVSRS(0, R5, R10)
>  	mffs	fr0
> -	stfd	fr0,FPSTATE_FPSCR(r8)
> +	stfd	fr0,FPSTATE_FPSCR(r10)
>  	PPC_LL	r5,PT_REGS(r4)
>  	toreal(r5)
>  	PPC_LL	r4,_MSR-STACK_FRAME_OVERHEAD(r5)
> @@ -157,10 +159,10 @@ END_FTR_SECTION_IFSET(CPU_FTR_VSX)
>  	or	r12,r12,r4
>  	std	r12,_MSR(r1)
>  #endif
> -	addi	r7,r5,THREAD_FPSTATE
> -	lfd	fr0,FPSTATE_FPSCR(r7)
> +	addi	r10,r5,THREAD_FPSTATE
> +	lfd	fr0,FPSTATE_FPSCR(r10)
>  	MTFSF_L(fr0)
> -	REST_32FPVSRS(0, R4, R7)
> +	REST_32FPVSRS(0, R4, R10)
>  #ifndef CONFIG_SMP
>  	subi	r4,r5,THREAD
>  	fromreal(r4)
> diff --git a/arch/powerpc/kernel/vector.S b/arch/powerpc/kernel/vector.S
> index eacda4e..0458a9a 100644
> --- a/arch/powerpc/kernel/vector.S
> +++ b/arch/powerpc/kernel/vector.S
> @@ -64,6 +64,9 @@ _GLOBAL(store_vr_state)
>   * Enables the VMX for use in the kernel on return.
>   * On SMP we know the VMX is free, since we give it up every
>   * switch (ie, no lazy save of the vector registers).
> + *
> + * Note that on 32-bit this can only use registers that will be
> + * restored by fast_exception_return, i.e. r3 - r6, r10 and r11.
>   */
>  _GLOBAL(load_up_altivec)
>  	mfmsr	r5			/* grab the current MSR */
> @@ -89,11 +92,11 @@ _GLOBAL(load_up_altivec)
>  	/* Save VMX state to last_task_used_altivec's THREAD struct */
>  	toreal(r4)
>  	addi	r4,r4,THREAD
> -	addi	r7,r4,THREAD_VRSTATE
> -	SAVE_32VRS(0,r5,r7)
> +	addi	r6,r4,THREAD_VRSTATE
> +	SAVE_32VRS(0,r5,r6)
>  	mfvscr	vr0
>  	li	r10,VRSTATE_VSCR
> -	stvx	vr0,r10,r7
> +	stvx	vr0,r10,r6
>  	/* Disable VMX for last_task_used_altivec */
>  	PPC_LL	r5,PT_REGS(r4)
>  	toreal(r5)
> @@ -125,13 +128,13 @@ _GLOBAL(load_up_altivec)
>  	oris	r12,r12,MSR_VEC@h
>  	std	r12,_MSR(r1)
>  #endif
> -	addi	r7,r5,THREAD_VRSTATE
> +	addi	r6,r5,THREAD_VRSTATE
>  	li	r4,1
>  	li	r10,VRSTATE_VSCR
>  	stw	r4,THREAD_USED_VR(r5)
> -	lvx	vr0,r10,r7
> +	lvx	vr0,r10,r6
>  	mtvscr	vr0
> -	REST_32VRS(0,r4,r7)
> +	REST_32VRS(0,r4,r6)
>  #ifndef CONFIG_SMP
>  	/* Update last_task_used_altivec to 'current' */
>  	subi	r4,r5,THREAD		/* Back to 'current' */
> -- 
> 1.8.4.rc3
> 
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By any chance, same corruption is happening for DEBUG_DEBUG exception ?
because I could see similar SEGV but dont have any code/program to prove it
:(



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