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

From: "Jason A. Donenfeld" <[email-masked]>
Date: Fri, 6 Sep 2024 05:24:24 +0200

On Fri, Sep 06, 2024 at 04:48:28AM +0200, Jason A. Donenfeld wrote:
> On Thu, Sep 05, 2024 at 10:41:40PM +0200, Jason A. Donenfeld wrote:
> > 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.
> 
> Oh, I just got one small detail wrong before. The below actually works,
> and uses the same strategy as on arm64.
> 
> Let me know if you'd like me to fix up this commit with the below patch,
> or if you have another way you'd like to go about it.

And here's the much shorter version in assembly, which maybe you prefer.
Also works, and is a bit less invasive than the other thing.

diff --git a/arch/powerpc/kernel/vdso/getrandom.S b/arch/powerpc/kernel/vdso/getrandom.S
index a957cd2b2b03..070daba2d547 100644
--- a/arch/powerpc/kernel/vdso/getrandom.S
+++ b/arch/powerpc/kernel/vdso/getrandom.S
@@ -32,6 +32,14 @@
   .cfi_rel_offset r2, PPC_MIN_STKFRM + STK_GOT
 #endif
 	get_datapage	r8
+#ifdef CONFIG_TIME_NS
+	lis		r10, 0x7fff
+	ori		r10, r10, 0xffff
+	lwz		r9, VDSO_DATA_OFFSET + 4(r8)
+	cmpw		r9, r10
+	bne		+8
+	addi		r8, r8, (1 << CONFIG_PAGE_SHIFT)
+#endif
 	addi		r8, r8, VDSO_RNG_DATA_OFFSET
 	bl		CFUNC(DOTSYM(\funct))
 	PPC_LL		r0, PPC_MIN_STKFRM + PPC_LR_STKOFF(r1)