Re: [PATCH 9/9] ARM: add uprobes support

[Rabin Vincent]

2012/10/15 Dave Martin <dave.martin@xxxxxxxxxx>:
> On Sun, Oct 14, 2012 at 09:23:13PM +0200, Rabin Vincent wrote:
>> Add basic uprobes support for ARM.
>>
>> perf probe --exec and SystemTap's userspace probing work.  The ARM
>> kprobes test code has also been run in a userspace harness to test the
>> uprobe instruction decoding.
>
> The assumption that the target code is ARM appears to be buried all over
> the place.

Right, as stated:

>> Caveats:
>>  - Thumb is not supported

> Certainly this code as currently written must depend on !THUMB2_KERNEL.

Why?  It currently works for ARM userspace even if the kernel is
Thumb-2.

> However, there's an underlying problem here which we'd need to solve.
> The kprobes code can take advantage of the fact that the kernel is all
> ARM or (almost) all Thumb code.  So there is no support for kprobes
> supporting ARM and Thumb at the same time.
>
> With userspace, we don't have this luxury.  With Debian armhf, Ubuntu
> and Linaro building Thumb-2 userspaces, it may be an increasingly common
> configuration independently of whether the kernel is built in Thumb-2
> or not.
>
> Furthermode, there is no such thing as a pure Thumb-2 system in practice:
> PLTs are always ARM, for example.  For uprobes to work well in userspace,
> both should be supported together.

Right.  I don't think it's difficult to support both of them together,
my thought was just to have userspace tell us if they want to probe a
Thumb instruction via the usual 0th-bit set convention, and then take it
from there.  I didn't do the Thumb handling currently because I didn't
really look into modifying the kprobes Thumb instruction decoding and
the IT handling for uprobes.
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