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

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Date:   Fri, 19 Apr 2019 14:33:10 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.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, 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!