Xu Kuohai xukuohai@huaweicloud.com 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#paramet...
Signed-off-by: Puranjay Mohan puranjay@kernel.org
Changes in V1 -> V2: V1: https://lore.kernel.org/all/20240704173227.130491-1-puranjay@kernel.org/
- 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.
Alsoi, do we currently have a selftest that checks for this edge case? if not I can try to add that too.
Thanks, Puranjay
[1] https://lore.kernel.org/bpf/20230917150752.69612-1-xukuohai@huaweicloud.com/ [2] https://lore.kernel.org/bpf/CABRcYmLtk8aQEzoUFw+j5Rdd-MXf-q+i7RHXZtu-skjRz11...