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Re: [x86-64 psABI] RFC: Extend x86-64 PLT entry to support MPX
- From: "H.J. Lu" <hjl dot tools at gmail dot com>
- To: Ian Lance Taylor <iant at google dot com>
- Cc: GNU C Library <libc-alpha at sourceware dot org>, GCC Development <gcc at gcc dot gnu dot org>, Binutils <binutils at sourceware dot org>, "Girkar, Milind" <milind dot girkar at intel dot com>, "Kreitzer, David L" <david dot l dot kreitzer at intel dot com>
- Date: Wed, 24 Jul 2013 11:53:20 -0700
- Subject: Re: [x86-64 psABI] RFC: Extend x86-64 PLT entry to support MPX
- References: <CAMe9rOp=1v38F_aV-pbv50YOGSEr_ju+byZP1L_G_h4bm5Ad3w at mail dot gmail dot com> <CAKOQZ8y-8Q7f063mM4zKN3QDCLBAqng-9xNVYoARD8uR_K297A at mail dot gmail dot com>
On Wed, Jul 24, 2013 at 9:45 AM, Ian Lance Taylor <email@example.com> wrote:
> On Tue, Jul 23, 2013 at 12:49 PM, H.J. Lu <firstname.lastname@example.org> wrote:
>> introduces 4 bound registers, which will be used for parameter passing
>> in x86-64. Bound registers are cleared by branch instructions. Branch
>> instructions with BND prefix will keep bound register contents.
> I took a very quick look at the doc. Why shouldn't we run the kernel
> with BNDPRESERVE = 1, to avoid this behaviour of clearing the bound
> registers on branch instructions? That would let us avoid these
This doesn't work in case of legacy callees which return pointers.
The bound registers will be incorrect since they are set in the
last MPX function. MPX callers will get wrong bounds on
pointers returned by legacy callees
>> I prefer the note section solution. Any suggestions, comments?
> I concur, but why not use the ELF attributes support rather than a new
> note section?
The issues are
1. ELF attributes target static linker. There is no support in
shared library nor executables. We may need it to make run-time
decision based on MPX feature to select legacy or MPX share
2. ELF attribute lookup isn't very fast at run-time.