[PATCH 4/9] powerpc/bpf: Handle large branch ranges with BPF_EXIT

[Christophe Leroy]

Le 01/10/2021 à 23:14, Naveen N. Rao a écrit :
> In some scenarios, it is possible that the program epilogue is outside
> the branch range for a BPF_EXIT instruction. Instead of rejecting such
> programs, emit an indirect branch. We track the size of the bpf program
> emitted after the initial run and do a second pass since BPF_EXIT can
> end up emitting different number of instructions depending on the
> program size.
> 
> Suggested-by: Jordan Niethe <jniethe5 at gmail.com>
> Signed-off-by: Naveen N. Rao <naveen.n.rao at linux.vnet.ibm.com>
> ---
>   arch/powerpc/net/bpf_jit.h        |  3 +++
>   arch/powerpc/net/bpf_jit_comp.c   | 22 +++++++++++++++++++++-
>   arch/powerpc/net/bpf_jit_comp32.c |  2 +-
>   arch/powerpc/net/bpf_jit_comp64.c |  2 +-
>   4 files changed, 26 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/powerpc/net/bpf_jit.h b/arch/powerpc/net/bpf_jit.h
> index 89bd744c2bffd4..4023de1698b9f5 100644
> --- a/arch/powerpc/net/bpf_jit.h
> +++ b/arch/powerpc/net/bpf_jit.h
> @@ -126,6 +126,7 @@
>   
>   #define SEEN_FUNC	0x20000000 /* might call external helpers */
>   #define SEEN_TAILCALL	0x40000000 /* uses tail calls */
> +#define SEEN_BIG_PROG	0x80000000 /* large prog, >32MB */
>   
>   #define SEEN_VREG_MASK	0x1ff80000 /* Volatile registers r3-r12 */
>   #define SEEN_NVREG_MASK	0x0003ffff /* Non volatile registers r14-r31 */
> @@ -179,6 +180,8 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *
>   void bpf_jit_build_prologue(u32 *image, struct codegen_context *ctx);
>   void bpf_jit_build_epilogue(u32 *image, struct codegen_context *ctx);
>   void bpf_jit_realloc_regs(struct codegen_context *ctx);
> +int bpf_jit_emit_exit_insn(u32 *image, struct codegen_context *ctx,
> +					int tmp_reg, unsigned long exit_addr);
>   
>   #endif
>   
> diff --git a/arch/powerpc/net/bpf_jit_comp.c b/arch/powerpc/net/bpf_jit_comp.c
> index fcbf7a917c566e..3204872fbf2738 100644
> --- a/arch/powerpc/net/bpf_jit_comp.c
> +++ b/arch/powerpc/net/bpf_jit_comp.c
> @@ -72,6 +72,21 @@ static int bpf_jit_fixup_subprog_calls(struct bpf_prog *fp, u32 *image,
>   	return 0;
>   }
>   
> +int bpf_jit_emit_exit_insn(u32 *image, struct codegen_context *ctx,
> +					int tmp_reg, unsigned long exit_addr)
> +{
> +	if (!(ctx->seen & SEEN_BIG_PROG) && is_offset_in_branch_range(exit_addr)) {
> +		PPC_JMP(exit_addr);
> +	} else {
> +		ctx->seen |= SEEN_BIG_PROG;
> +		PPC_FUNC_ADDR(tmp_reg, (unsigned long)image + exit_addr);
> +		EMIT(PPC_RAW_MTCTR(tmp_reg));
> +		EMIT(PPC_RAW_BCTR());
> +	}
> +
> +	return 0;
> +}
> +
>   struct powerpc64_jit_data {
>   	struct bpf_binary_header *header;
>   	u32 *addrs;
> @@ -155,12 +170,17 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *fp)
>   		goto out_addrs;
>   	}
>   
> +	if (!is_offset_in_branch_range((long)cgctx.idx * 4))
> +		cgctx.seen |= SEEN_BIG_PROG;
> +
>   	/*
>   	 * If we have seen a tail call, we need a second pass.
>   	 * This is because bpf_jit_emit_common_epilogue() is called
>   	 * from bpf_jit_emit_tail_call() with a not yet stable ctx->seen.
> +	 * We also need a second pass if we ended up with too large
> +	 * a program so as to fix branches.
>   	 */
> -	if (cgctx.seen & SEEN_TAILCALL) {
> +	if (cgctx.seen & (SEEN_TAILCALL | SEEN_BIG_PROG)) {
>   		cgctx.idx = 0;
>   		if (bpf_jit_build_body(fp, 0, &cgctx, addrs, false)) {
>   			fp = org_fp;
> diff --git a/arch/powerpc/net/bpf_jit_comp32.c b/arch/powerpc/net/bpf_jit_comp32.c
> index a74d52204f8da2..d2a67574a23066 100644
> --- a/arch/powerpc/net/bpf_jit_comp32.c
> +++ b/arch/powerpc/net/bpf_jit_comp32.c
> @@ -852,7 +852,7 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *
>   			 * we'll just fall through to the epilogue.
>   			 */
>   			if (i != flen - 1)
> -				PPC_JMP(exit_addr);
> +				bpf_jit_emit_exit_insn(image, ctx, tmp_reg, exit_addr);

On ppc32, if you use tmp_reg you must flag it. But I think you could use 
r0 instead.

>   			/* else fall through to the epilogue */
>   			break;
>   
> diff --git a/arch/powerpc/net/bpf_jit_comp64.c b/arch/powerpc/net/bpf_jit_comp64.c
> index f06c62089b1457..3351a866ef6207 100644
> --- a/arch/powerpc/net/bpf_jit_comp64.c
> +++ b/arch/powerpc/net/bpf_jit_comp64.c
> @@ -761,7 +761,7 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *
>   			 * we'll just fall through to the epilogue.
>   			 */
>   			if (i != flen - 1)
> -				PPC_JMP(exit_addr);
> +				bpf_jit_emit_exit_insn(image, ctx, b2p[TMP_REG_1], exit_addr);
>   			/* else fall through to the epilogue */
>   			break;
>   
>