netdev - Re: [PATCH v4 bpf-next 02/15] bpf: mark lo32 writes that should be zero extended into hi32

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Message-ID: <20190419214121.2rbmmms6qozmiuke@ast-mbp.dhcp.thefacebook.com>
Date:   Fri, 19 Apr 2019 14:41:23 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>
Cc:     Jiong Wang <jiong.wang@...ronome.com>, daniel@...earbox.net,
        bpf@...r.kernel.org, netdev@...r.kernel.org,
        oss-drivers@...ronome.com
Subject: Re: [PATCH v4 bpf-next 02/15] bpf: mark lo32 writes that should be
 zero extended into hi32

On Fri, Apr 19, 2019 at 02:33:10PM -0700, Jakub Kicinski wrote:
> On Fri, 19 Apr 2019 14:14:05 -0700, Alexei Starovoitov wrote:
> > > This reminds me, I'm not entirely clear on the need to propagate the
> > > zext through stack slots...  Pointers are guaranteed to be 64bit, we
> > > don't save parentage on scalars (AFAICT),  
> > 
> > scalars have parentage chain too.
> > we don't track them precisely when they're spilled to stack.
> > That actually caused an issue recently when valid program was rejected,
> > so we might add a feature to track full contents of scalars in the stack.
> 
> Interesting..
> 
> > > why not pass REG_LIVE_READ
> > > or READ64 to mark_reg_read() from stack_read?  
> > 
> > can we agree on only two states first ? ;)
> 
> Yess, the LIVE_READ was thought to be more of a mask for those accesses
> that only care about "any read" being set, to be honest.  As you said
> read64 is a strict superset of read32.  Keeping the name REG_LIVE_READ,
> rather than REG_LIVE_READ_ANY or _MASK let us leave some of the
> existing code untouched.
> 
> Jiong's original idea was to add a read32, and have read mean read64.
> 
> I think you said we should have read32 and read64 flags, but clear
> read32 once read64 gets set?  SGTM!

yep.

any subsequent read64 means that earlier read32 marks are irrelevant
from zext optimization pov.