Re: [PATCH bpf-next v9 3/4] bpf, arm64: Implement bpf_arch_text_poke() for arm64

[Jon Hunter]

On 11/07/2022 16:08, Xu Kuohai wrote:
> Implement bpf_arch_text_poke() for arm64, so bpf prog or bpf trampoline
> can be patched with it.
>
> When the target address is NULL, the original instruction is patched to a
> NOP.
>
> When the target address and the source address are within the branch
> range, the original instruction is patched to a bl instruction to the
> target address directly.
>
> To support attaching bpf trampoline to both regular kernel function and
> bpf prog, we follow the ftrace patchsite way for bpf prog. That is, two
> instructions are inserted at the beginning of bpf prog, the first one
> saves the return address to x9, and the second is a nop which will be
> patched to a bl instruction when a bpf trampoline is attached.
>
> However, when a bpf trampoline is attached to bpf prog, the distance
> between target address and source address may exceed 128MB, the maximum
> branch range, because bpf trampoline and bpf prog are allocated
> separately with vmalloc. So long jump should be handled.
>
> When a bpf prog is constructed, a plt pointing to empty trampoline
> dummy_tramp is placed at the end:
>
>          bpf_prog:
>                  mov x9, lr
>                  nop // patchsite
>                  ...
>                  ret
>
>          plt:
>                  ldr x10, target
>                  br x10
>          target:
>                  .quad dummy_tramp // plt target
>
> This is also the state when no trampoline is attached.
>
> When a short-jump bpf trampoline is attached, the patchsite is patched to
> a bl instruction to the trampoline directly:
>
>          bpf_prog:
>                  mov x9, lr
>                  bl <short-jump bpf trampoline address> // patchsite
>                  ...
>                  ret
>
>          plt:
>                  ldr x10, target
>                  br x10
>          target:
>                  .quad dummy_tramp // plt target
>
> When a long-jump bpf trampoline is attached, the plt target is filled with
> the trampoline address and the patchsite is patched to a bl instruction to
> the plt:
>
>          bpf_prog:
>                  mov x9, lr
>                  bl plt // patchsite
>                  ...
>                  ret
>
>          plt:
>                  ldr x10, target
>                  br x10
>          target:
>                  .quad <long-jump bpf trampoline address>
>
> dummy_tramp is used to prevent another CPU from jumping to an unknown
> location during the patching process, making the patching process easier.
>
> The patching process is as follows:
>
> 1. when neither the old address or the new address is a long jump, the
>     patchsite is replaced with a bl to the new address, or nop if the new
>     address is NULL;
>
> 2. when the old address is not long jump but the new one is, the
>     branch target address is written to plt first, then the patchsite
>     is replaced with a bl instruction to the plt;
>
> 3. when the old address is long jump but the new one is not, the address
>     of dummy_tramp is written to plt first, then the patchsite is replaced
>     with a bl to the new address, or a nop if the new address is NULL;
>
> 4. when both the old address and the new address are long jump, the
>     new address is written to plt and the patchsite is not changed.
>
> Signed-off-by: Xu Kuohai <xukuohai@xxxxxxxxxx>
> Acked-by: Song Liu <songliubraving@xxxxxx>
> Reviewed-by: Jakub Sitnicki <jakub@xxxxxxxxxxxxxx>
> Reviewed-by: KP Singh <kpsingh@xxxxxxxxxx>
> Reviewed-by: Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx>
> ---
>   arch/arm64/net/bpf_jit.h      |   7 +
>   arch/arm64/net/bpf_jit_comp.c | 329 ++++++++++++++++++++++++++++++++--
>   2 files changed, 322 insertions(+), 14 deletions(-)


This change appears to be causing the build to fail ...

/tmp/cc52xO0c.s: Assembler messages:
/tmp/cc52xO0c.s:8: Error: operand 1 should be an integer register -- `mov lr,x9'
/tmp/cc52xO0c.s:7: Error: undefined symbol lr used as an immediate value
make[2]: *** [scripts/Makefile.build:250: arch/arm64/net/bpf_jit_comp.o] Error 1
make[1]: *** [scripts/Makefile.build:525: arch/arm64/net] Error 2

Let me know if you have any thoughts.

Cheers
Jon

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