Re: [PATCH bpf-next v2] arm64, bpf: Add 12-argument support for bpf trampoline

[Xu Kuohai]

On 7/8/2024 5:00 PM, Puranjay Mohan wrote:
> Xu Kuohai <xukuohai@xxxxxxxxxxxxxxx> writes:
>
> > On 7/5/2024 8:53 PM, Puranjay Mohan wrote:
> > > The arm64 bpf JIT currently supports attaching the trampoline to
> > > functions with <= 8 arguments. This is because up to 8 arguments can be
> > > passed in registers r0-r7. If there are more than 8 arguments then the
> > > 9th and later arguments are passed on the stack, with SP pointing to the
> > > first stacked argument. See aapcs64[1] for more details.
> > >
> > > If the 8th argument is a structure of size > 8B, then it is passed fully
> > > on stack and r7 is not used for passing any argument. If there is a 9th
> > > argument, it will be passed on the stack, even though r7 is available.
> > >
> > > Add the support of storing and restoring arguments passed on the stack
> > > to the arm64 bpf trampoline. This will allow attaching the trampoline to
> > > functions that take up to 12 arguments.
> > >
> > > [1] https://github.com/ARM-software/abi-aa/blob/main/aapcs64/aapcs64.rst#parameter-passing
> > >
> > > Signed-off-by: Puranjay Mohan <puranjay@xxxxxxxxxx>
> > > ---
> > > Changes in V1 -> V2:
> > > V1: https://lore.kernel.org/all/20240704173227.130491-1-puranjay@xxxxxxxxxx/
> > > - Fixed the argument handling for composite types (structs)
> > > ---
> > >    arch/arm64/net/bpf_jit_comp.c                | 139 ++++++++++++++-----
> > >    tools/testing/selftests/bpf/DENYLIST.aarch64 |   3 -
> > >    2 files changed, 107 insertions(+), 35 deletions(-)
>
> [SNIP]
>
> > >    fill_link_info/kprobe_multi_invalid_ubuff        # bpf_program__attach_kprobe_multi_opts unexpected error: -95
> >
> > It looks like this patch, similar to [1], also does not handle
> > parameter alignment properly [2].
> >
> > For example:
> >
> > int func(__int128 a, __int128 b, __int128 c, int64_t d, __int128 e, int64_t f, __int128 g)
> > {
> > }
> >
> > parameter a~d are passed through register x0~x6, while parameter
> > e~g are passed through stack. Since __int128 is 16-byte aligned,
> > parameter e, f, and g should be placed at sp + 0, +16, and +32
> > respectively, with 8 bytes **padding** between f and g.
> >
> >
> > And the compiler's alignment or packed attribute may make things
> > worse, causing parameters to be placed on the stack at positions
> > that are not naturally aligned.
>
> Hi Xu,
> Thanks for explaining this. I was not aware that you have already sent a
> patch[1] to add this support to arm64.
>
> So, I see that it will be non-trivial to calculate padding for each
> argument passed on the stack. If you are not planning to work on this
> then I can try to implement it.

Sure, go ahead and do it.

> Alsoi, do we currently have a selftest that checks for this edge case? if
> not I can try to add that too.

Not yet, feel free to add it. Thanks.

> Thanks,
> Puranjay
>
> > [1] https://lore.kernel.org/bpf/20230917150752.69612-1-xukuohai@xxxxxxxxxxxxxxx/
> > [2] https://lore.kernel.org/bpf/CABRcYmLtk8aQEzoUFw+j5Rdd-MXf-q+i7RHXZtu-skjRz11ZDw@xxxxxxxxxxxxxx/