Re: ARM ABI calling conventions

[Wade Farnsworth <wade_farnsworth at mentor dot com>]

Mark Wielaard wrote:
> Hi,
>
> I need some help with ARM calling conventions. While cleaning up some
> arm support code (I am now able to get nice backtraces across
> modules/kernel boundaries using the dwarf unwinder, whoho!), I noticed
> the following failures:
>
> Running /root/systemtap/testsuite/systemtap.context/context.exp ...
> FAIL: int64 function arguments -- numeric
> FAIL: int64 function arguments -- numeric --kelf --ignore-dwarf
>
> Which is now http://sourceware.org/bugzilla/show_bug.cgi?id=13466
>
> The problem is that I don't fully understand which ABI calling
> conventions are in used on the various ARM distros. Nor do I understand
> how to detect which one is being used. Can the ABI calling conventions
> be different between kernel and user space?

Yes, in theory the ABI calling conventions can differ between kernel and 
user space as well as between individual user space binaries.

There are a few ways to detect the ABI option used in a particular 
binary. See http://wiki.debian.org/ArmEabiPort .

> Fedora15 ARM HardFP port [*] uses EABI (though it isn't officially
> called EABI anymore). That is what I am using on a little trimslice now.
> But apparently older Fedora ports also used EABI (at least for the
> integral argument passing, floating point argument passing is a
> different story).
>
> What tapsets/arm/register.stp seems to support is the ARM OABI.
> Are there still distros that use that one?

I expect most distributions have switched to EABI by now, though it 
wouldn't surprise me if there was some legacy code running somewhere 
that uses OABI.

Would it be possible to support both ABIs, parsing the ELF header to 
obtain the flag and then take the appropriate action?

-Wade

> Thanks,
>
> Mark
>
> [*]
> https://fedoraproject.org/wiki/Architectures/ARM/Fedora15_HardFP_Bootstrap
> which uses the calling convention described in the AAPCS
> http://infocenter.arm.com/help/topic/com.arm.doc.ihi0042d/IHI0042D_aapcs.pdf