netdev - Re: [PATCH bpf-next v3 1/3] bpf, x86: allow function arguments up to 12 for TRACING

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Date: Wed, 7 Jun 2023 13:09:05 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: menglong8.dong@...il.com
Cc: davem@...emloft.net, dsahern@...nel.org, ast@...nel.org,
	daniel@...earbox.net, andrii@...nel.org, martin.lau@...ux.dev,
	song@...nel.org, yhs@...com, john.fastabend@...il.com,
	kpsingh@...nel.org, sdf@...gle.com, x86@...nel.org,
	imagedong@...cent.com, benbjiang@...cent.com,
	netdev@...r.kernel.org, bpf@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH bpf-next v3 1/3] bpf, x86: allow function arguments up to
 12 for TRACING

On Wed, Jun 07, 2023 at 08:59:09PM +0800, menglong8.dong@...il.com wrote:
> From: Menglong Dong <imagedong@...cent.com>
> 
> For now, the BPF program of type BPF_PROG_TYPE_TRACING can only be used
> on the kernel functions whose arguments count less than 6. This is not
> friendly at all, as too many functions have arguments count more than 6.
> 
> Therefore, let's enhance it by increasing the function arguments count
> allowed in arch_prepare_bpf_trampoline(), for now, only x86_64.
> 
> For the case that we don't need to call origin function, which means
> without BPF_TRAMP_F_CALL_ORIG, we need only copy the function arguments
> that stored in the frame of the caller to current frame. The arguments
> of arg6-argN are stored in "$rbp + 0x18", we need copy them to
> "$rbp - regs_off + (6 * 8)".
> 
> For the case with BPF_TRAMP_F_CALL_ORIG, we need prepare the arguments
> in stack before call origin function, which means we need alloc extra
> "8 * (arg_count - 6)" memory in the top of the stack. Note, there should
> not be any data be pushed to the stack before call the origin function.
> Then, we have to store rbx with 'mov' instead of 'push'.

x86-64 psABI requires stack to be 16-byte aligned when args are passed on the stack.
I don't see this logic in the patch.