Subject: Re: [PATCH v5 4/5] powerpc/vdso: Wire up getrandom() vDSO implementation on VDSO32

From: "Jason A. Donenfeld" <[email-masked]>
Date: Thu, 5 Sep 2024 22:41:40 +0200

On Thu, Sep 05, 2024 at 06:13:29PM +0200, Jason A. Donenfeld wrote:
> > +/*
> > + * The macro sets two stack frames, one for the caller and one for the callee
> > + * because there are no requirement for the caller to set a stack frame when
> > + * calling VDSO so it may have omitted to set one, especially on PPC64
> > + */
> > +
> > +.macro cvdso_call funct
> > +  .cfi_startproc
> > +	PPC_STLU	r1, -PPC_MIN_STKFRM(r1)
> > +  .cfi_adjust_cfa_offset PPC_MIN_STKFRM
> > +	mflr		r0
> > +	PPC_STLU	r1, -PPC_MIN_STKFRM(r1)
> > +  .cfi_adjust_cfa_offset PPC_MIN_STKFRM
> > +	PPC_STL		r0, PPC_MIN_STKFRM + PPC_LR_STKOFF(r1)
> > +  .cfi_rel_offset lr, PPC_MIN_STKFRM + PPC_LR_STKOFF
> > +	get_datapage	r8
> > +	addi		r8, r8, VDSO_RNG_DATA_OFFSET
> > +	bl		CFUNC(DOTSYM(\funct))
> > +	PPC_LL		r0, PPC_MIN_STKFRM + PPC_LR_STKOFF(r1)
> > +	cmpwi		r3, 0
> > +	mtlr		r0
> > +	addi		r1, r1, 2 * PPC_MIN_STKFRM
> > +  .cfi_restore lr
> > +  .cfi_def_cfa_offset 0
> > +	crclr		so
> > +	bgelr+
> > +	crset		so
> > +	neg		r3, r3
> > +	blr
> > +  .cfi_endproc
> > +.endm
> 
> Can you figure out what's going on and send a fix, which I'll squash
> into this commit?

This doesn't work, but I wonder if something like it is what we want. I
need to head out for the day, but here's what I've got. It's all wrong
but might be of interest.

diff --git a/arch/powerpc/include/asm/vdso/getrandom.h b/arch/powerpc/include/asm/vdso/getrandom.h
index 501d6bb14e8a..acb271709d30 100644
--- a/arch/powerpc/include/asm/vdso/getrandom.h
+++ b/arch/powerpc/include/asm/vdso/getrandom.h
@@ -47,7 +47,8 @@ static __always_inline struct vdso_rng_data *__arch_get_vdso_rng_data(void)
 }

 ssize_t __c_kernel_getrandom(void *buffer, size_t len, unsigned int flags, void *opaque_state,
-			     size_t opaque_len, const struct vdso_rng_data *vd);
+			     size_t opaque_len, const struct vdso_data *vd,
+			     const struct vdso_rng_data *vrd);

 #endif /* !__ASSEMBLY__ */

diff --git a/arch/powerpc/kernel/vdso/getrandom.S b/arch/powerpc/kernel/vdso/getrandom.S
index a957cd2b2b03..bc49eb87cfd1 100644
--- a/arch/powerpc/kernel/vdso/getrandom.S
+++ b/arch/powerpc/kernel/vdso/getrandom.S
@@ -32,7 +32,7 @@
   .cfi_rel_offset r2, PPC_MIN_STKFRM + STK_GOT
 #endif
 	get_datapage	r8
-	addi		r8, r8, VDSO_RNG_DATA_OFFSET
+	addi		r9, r8, VDSO_RNG_DATA_OFFSET
 	bl		CFUNC(DOTSYM(\funct))
 	PPC_LL		r0, PPC_MIN_STKFRM + PPC_LR_STKOFF(r1)
 #ifdef __powerpc64__
diff --git a/arch/powerpc/kernel/vdso/vgetrandom.c b/arch/powerpc/kernel/vdso/vgetrandom.c
index 5f855d45fb7b..408c76036868 100644
--- a/arch/powerpc/kernel/vdso/vgetrandom.c
+++ b/arch/powerpc/kernel/vdso/vgetrandom.c
@@ -8,7 +8,10 @@
 #include <linux/types.h>

 ssize_t __c_kernel_getrandom(void *buffer, size_t len, unsigned int flags, void *opaque_state,
-			     size_t opaque_len, const struct vdso_rng_data *vd)
+			     size_t opaque_len, const struct vdso_data *vd,
+			     const struct vdso_rng_data *vrd)
 {
-	return __cvdso_getrandom_data(vd, buffer, len, flags, opaque_state, opaque_len);
+	if (IS_ENABLED(CONFIG_TIME_NS) && vd->clock_mode == VDSO_CLOCKMODE_TIMENS)
+		vrd = (void *)vrd + (1UL << CONFIG_PAGE_SHIFT);
+	return __cvdso_getrandom_data(vrd, buffer, len, flags, opaque_state, opaque_len);
 }