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. 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@xxxxxxxxxxxxxxx/ > [2] https://lore.kernel.org/bpf/CABRcYmLtk8aQEzoUFw+j5Rdd-MXf-q+i7RHXZtu-skjRz11ZDw@xxxxxxxxxxxxxx/
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