Re: [PATCH 2/3] fix matching errors related to i386 addressing modes in CFI generation script

[Alex <alexinbeijing@gmail.com>]

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On Mon, Oct 12, 2015 at 5:12 PM, Rich Felker <dalias@libc.org> wrote:

> On Mon, Oct 12, 2015 at 03:58:59PM +0200, Alex Dowad wrote:
> > the regexps previously used to identify registers clobbered by MOVs,
> ADDs,
> > and various other operations would erroneously match index registers. In
> other
> > words, the following asm:
> >
> >     mov $0, (%eax,%ebx,4)
> >
> > ....would cause EBX to be considered as overwritten, which might prevent
> a
> > debugger from displaying a variable's value in a higher stack frame.
> >
> > thanks to Rich Felker for noticing this problem.
> > ---
> >  tools/add-cfi.i386.awk | 10 +++++-----
> >  1 file changed, 5 insertions(+), 5 deletions(-)
> >
> > diff --git a/tools/add-cfi.i386.awk b/tools/add-cfi.i386.awk
> > index fc0d8cf..bd7932f 100644
> > --- a/tools/add-cfi.i386.awk
> > +++ b/tools/add-cfi.i386.awk
> > @@ -184,13 +184,13 @@ function trashed(register) {
> >  }
> >  # this does NOT exhaustively check for all possible instructions which
> could
> >  # overwrite a register value inherited from the caller (just the common
> ones)
> > -/mov.*,%e(ax|bx|cx|dx|si|di|bp)/  { trashed(get_reg2()) }
> >
> -/(add|addl|sub|subl|and|or|xor|lea|sal|sar|shl|shr).*,%e(ax|bx|cx|dx|si|di|bp)/
> {
> > +/mov.*,%e(ax|bx|cx|dx|si|di|bp)$/  { trashed(get_reg2()) }
> >
> +/(add|addl|sub|subl|and|or|xor|lea|sal|sar|shl|shr).*,%e(ax|bx|cx|dx|si|di|bp)$/
> {
> >    trashed(get_reg2())
> >  }
> > -/^i?mul [^,]*$/                     { trashed("eax"); trashed("edx") }
> > -/^i?mul.*,%e(ax|bx|cx|dx|si|di|bp)/ { trashed(get_reg2()) }
> > -/^i?div/                            { trashed("eax"); trashed("edx") }
> > +/^i?mul [^,]*$/                      { trashed("eax"); trashed("edx") }
> > +/^i?mul.*,%e(ax|bx|cx|dx|si|di|bp)$/ { trashed(get_reg2()) }
> > +/^i?div/                             { trashed("eax"); trashed("edx") }
> >  /(dec|inc|not|neg|pop) %e(ax|bx|cx|dx|si|di|bp)/  { trashed(get_reg()) }
> >  /cpuid/ { trashed("eax"); trashed("ebx"); trashed("ecx");
> trashed("edx") }
>
> Clever. At first I didn't see how this was fixing anything, with the
> .* still there, but given that you strip comments and extra
> whitespace, anchoring to the end with $ seems to work.
>
> While seeing them separately was useful for seeing how you fixed the
> bug, patches 1 and 2 should be merged for commit. All patch 2 is doing
> is fixing a bug that patch 1 introduces; together they just form a
> non-buggy version of "fix operand order". I can take care of the
> merging though.
>

Let me do it. I'll merge them and send another patch series.

One other thing I noticed for future improvement: your patterns don't
> seem to catch instructions that modify just the low byte or half of a
> register. These are fairly uncommon in musl's i386 asm, but for
> x86_64, I would estimate a good 50% of register usage uses the 32-bit
> half (%e..) of a register rather than the full %r.., and your current
> script fails to mark these clobbers at all. Probably the regex should
> be something like %[er]?([abcd][xlh]|si|di|bp|...) - I don't recall
> the right form for the numbered x86_64 registers' low parts right off,
> though.


Thanks for bringing this out. I'll fix this too and send again.

Alex

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