Re: [PATCH v4 bpf-next 9/9] selftests/bpf: Test epilogue patching when the main prog has multiple BPF_EXIT

[Martin KaFai Lau <martin.lau@linux.dev>]

On 8/27/24 12:48 PM, Martin KaFai Lau wrote:
> From: Martin KaFai Lau <martin.lau@kernel.org>
> 
> This patch tests the epilogue patching when the main prog has
> multiple BPF_EXIT. The verifier should have patched the 2nd (and
> later) BPF_EXIT with a BPF_JA that goes back to the earlier
> patched epilogue instructions.
> 
> Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
> ---
>   .../selftests/bpf/prog_tests/pro_epilogue.c   |  2 +
>   .../selftests/bpf/progs/epilogue_exit.c       | 78 +++++++++++++++++++
>   2 files changed, 80 insertions(+)
>   create mode 100644 tools/testing/selftests/bpf/progs/epilogue_exit.c
> 
> diff --git a/tools/testing/selftests/bpf/prog_tests/pro_epilogue.c b/tools/testing/selftests/bpf/prog_tests/pro_epilogue.c
> index b2e467cf15fe..58c18529a802 100644
> --- a/tools/testing/selftests/bpf/prog_tests/pro_epilogue.c
> +++ b/tools/testing/selftests/bpf/prog_tests/pro_epilogue.c
> @@ -6,6 +6,7 @@
>   #include "pro_epilogue_kfunc.skel.h"
>   #include "epilogue_tailcall.skel.h"
>   #include "pro_epilogue_goto_start.skel.h"
> +#include "epilogue_exit.skel.h"
>   
>   struct st_ops_args {
>   	int a;
> @@ -47,6 +48,7 @@ void test_pro_epilogue(void)
>   	RUN_TESTS(pro_epilogue_subprog);
>   	RUN_TESTS(pro_epilogue_kfunc);
>   	RUN_TESTS(pro_epilogue_goto_start);
> +	RUN_TESTS(epilogue_exit);
>   	if (test__start_subtest("tailcall"))
>   		test_tailcall();
>   }
> diff --git a/tools/testing/selftests/bpf/progs/epilogue_exit.c b/tools/testing/selftests/bpf/progs/epilogue_exit.c
> new file mode 100644
> index 000000000000..8c03256c7491
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/epilogue_exit.c
> @@ -0,0 +1,78 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Copyright (c) 2024 Meta Platforms, Inc. and affiliates. */
> +
> +#include <vmlinux.h>
> +#include <bpf/bpf_tracing.h>
> +#include "bpf_misc.h"
> +#include "../bpf_testmod/bpf_testmod.h"
> +#include "../bpf_testmod/bpf_testmod_kfunc.h"
> +
> +char _license[] SEC("license") = "GPL";
> +
> +__success
> +/* save __u64 *ctx to stack */
> +__xlated("0: *(u64 *)(r10 -8) = r1")
> +/* main prog */
> +__xlated("1: r1 = *(u64 *)(r1 +0)")
> +__xlated("2: r2 = *(u32 *)(r1 +0)")
> +__xlated("3: if r2 == 0x0 goto pc+10")
> +__xlated("4: r0 = 0")
> +__xlated("5: *(u32 *)(r1 +0) = 0")
> +/* epilogue */
> +__xlated("6: r1 = *(u64 *)(r10 -8)")
> +__xlated("7: r1 = *(u64 *)(r1 +0)")
> +__xlated("8: r6 = *(u32 *)(r1 +0)")
> +__xlated("9: w6 += 10000")
> +__xlated("10: *(u32 *)(r1 +0) = r6")
> +__xlated("11: w0 = w6")
> +__xlated("12: w0 *= 2")
> +__xlated("13: exit")
> +/* 2nd part of the main prog after the first exit */
> +__xlated("14: *(u32 *)(r1 +0) = 1")
> +__xlated("15: r0 = 1")
> +/* Clear the r1 to ensure it does not have
> + * off-by-1 error and ensure it jumps back to the
> + * beginning of epilogue which initializes
> + * the r1 with the ctx ptr.
> + */
> +__xlated("16: r1 = 0")
> +__xlated("17: gotol pc-12")
> +SEC("struct_ops/test_epilogue_exit")
> +__naked int test_epilogue_exit(void)
> +{
> +	asm volatile (
> +	"r1 = *(u64 *)(r1 +0);"
> +	"r2 = *(u32 *)(r1 +0);"
> +	"if r2 == 0 goto +3;"
> +	"r0 = 0;"
> +	"*(u32 *)(r1 + 0) = 0;"

llvm17 cannot take "*(u32 *)(r1 +0) = 0".

Instead:

r3 = 0;
*(u32 *)(r1 + 0) = r3;

The above solved the llvm17 error:
https://github.com/kernel-patches/bpf/actions/runs/10586206183/job/29334690461

However, there is still a zext with s390 that added extra insn and failed the 
__xlated check. will try an adjustment in the tests to avoid the zext.

pw-bot: cr


> +	"exit;"
> +	"*(u32 *)(r1 + 0) = 1;"
> +	"r0 = 1;"
> +	"r1 = 0;"
> +	"exit;"
> +	::: __clobber_all);
> +}
> +
> +SEC(".struct_ops.link")
> +struct bpf_testmod_st_ops epilogue_exit = {
> +	.test_epilogue = (void *)test_epilogue_exit,
> +};
> +
> +SEC("syscall")
> +__retval(20000)
> +int syscall_epilogue_exit0(void *ctx)
> +{
> +	struct st_ops_args args = { .a = 1 };
> +
> +	return bpf_kfunc_st_ops_test_epilogue(&args);
> +}
> +
> +SEC("syscall")
> +__retval(20002)
> +int syscall_epilogue_exit1(void *ctx)
> +{
> +	struct st_ops_args args = {};
> +
> +	return bpf_kfunc_st_ops_test_epilogue(&args);
> +}