[PATCH 02/13] powerpc32/bpf: Fix codegen for bpf-to-bpf calls

[Christophe Leroy]

Le 06/01/2022 à 12:45, Naveen N. Rao a écrit :
> Pad instructions emitted for BPF_CALL so that the number of instructions
> generated does not change for different function addresses. This is
> especially important for calls to other bpf functions, whose address
> will only be known during extra pass.

In first pass, 'image' is NULL and we emit the 4 instructions sequence 
already, so the code won't grow after first pass, it can only shrink.

On PPC32, a huge effort is made to minimise the situations where 'bl' 
cannot be used, see commit 2ec13df16704 ("powerpc/modules: Load modules 
closer to kernel text")

And if you take the 8xx for instance, a NOP a just like any other 
instruction, it takes one cycle.

If it is absolutely needed, then I'd prefer we use an out-of-line 
trampoline for the unlikely case and use 'bl' to that trampoline.

> 
> Fixes: 51c66ad849a703 ("powerpc/bpf: Implement extended BPF on PPC32")
> Cc: stable at vger.kernel.org # v5.13+
> Signed-off-by: Naveen N. Rao <naveen.n.rao at linux.vnet.ibm.com>
> ---
>   arch/powerpc/net/bpf_jit_comp32.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/arch/powerpc/net/bpf_jit_comp32.c b/arch/powerpc/net/bpf_jit_comp32.c
> index d3a52cd42f5346..997a47fa615b30 100644
> --- a/arch/powerpc/net/bpf_jit_comp32.c
> +++ b/arch/powerpc/net/bpf_jit_comp32.c
> @@ -191,6 +191,9 @@ void bpf_jit_emit_func_call_rel(u32 *image, struct codegen_context *ctx, u64 fun
>   
>   	if (image && rel < 0x2000000 && rel >= -0x2000000) {
>   		PPC_BL_ABS(func);
> +		EMIT(PPC_RAW_NOP());
> +		EMIT(PPC_RAW_NOP());
> +		EMIT(PPC_RAW_NOP());
>   	} else {
>   		/* Load function address into r0 */
>   		EMIT(PPC_RAW_LIS(_R0, IMM_H(func)));