Re: [PATCH bpf-next 6/8] bpf: Add arch_bpf_trampoline_size()

Xu Kuohai
2023-09-20T07:39:35Z

Hi Song,

On 9/20/2023 1:31 PM, Song Liu wrote:
> This helper will be used to calculate the size of the trampoline before
> allocating the memory.
>
> Signed-off-by: Song Liu <song@kernel.org>
> ---
> arch/arm64/net/bpf_jit_comp.c | 56 ++++++++++++++++++++++++---------
> arch/riscv/net/bpf_jit_comp64.c | 24 +++++++++-----
> arch/s390/net/bpf_jit_comp.c | 52 +++++++++++++++++-------------
> arch/x86/net/bpf_jit_comp.c | 40 ++++++++++++++++++++---
> include/linux/bpf.h | 2 ++
> kernel/bpf/trampoline.c | 6 ++++
> 6 files changed, 131 insertions(+), 49 deletions(-)
>
> diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
> index d81b886ea4df..a6671253b7ed 100644
> --- a/arch/arm64/net/bpf_jit_comp.c
> +++ b/arch/arm64/net/bpf_jit_comp.c
> @@ -2026,18 +2026,10 @@ static int prepare_trampoline(struct jit_ctx *ctx, struct bpf_tramp_image *im,
> return ctx->idx;
> }
>
> -int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image,
> - void *image_end, const struct btf_func_model *m,
> - u32 flags, struct bpf_tramp_links *tlinks,
> - void *func_addr)
> +static int btf_func_model_nregs(const struct btf_func_model *m)
> {
> - int i, ret;
> int nregs = m->nr_args;
> - int max_insns = ((long)image_end - (long)image) / AARCH64_INSN_SIZE;
> - struct jit_ctx ctx = {
> - .image = NULL,
> - .idx = 0,
> - };
> + int i;
>
> /* extra registers needed for struct argument */
> for (i = 0; i < MAX_BPF_FUNC_ARGS; i++) {
> @@ -2046,19 +2038,53 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image,
> nregs += (m->arg_size[i] + 7) / 8 - 1;
> }
>
> + return nregs;
> +}
> +
> +int arch_bpf_trampoline_size(const struct btf_func_model *m, u32 flags,
> + struct bpf_tramp_links *tlinks, void *func_addr)
> +{
> + struct jit_ctx ctx = {
> + .image = NULL,
> + .idx = 0,
> + };
> + struct bpf_tramp_image im;
> + int nregs, ret;
> +
> + nregs = btf_func_model_nregs(m);
> /* the first 8 registers are used for arguments */
> if (nregs > 8)
> return -ENOTSUPP;
>
> - ret = prepare_trampoline(&ctx, im, tlinks, func_addr, nregs, flags);
> + ret = prepare_trampoline(&ctx, &im, tlinks, func_addr, nregs, flags);
> if (ret < 0)
> return ret;
>
> - if (ret > max_insns)
> - return -EFBIG;
> + return ret < 0 ? ret : ret * AARCH64_INSN_SIZE;
> +}
>
> - ctx.image = image;
> - ctx.idx = 0;
> +int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image,
> + void *image_end, const struct btf_func_model *m,
> + u32 flags, struct bpf_tramp_links *tlinks,
> + void *func_addr)
> +{
> + int ret, nregs;
> + struct jit_ctx ctx = {
> + .image = image,
> + .idx = 0,
> + };
> +
> + nregs = btf_func_model_nregs(m);
> + /* the first 8 registers are used for arguments */
> + if (nregs > 8)
> + return -ENOTSUPP;
> +
> + ret = arch_bpf_trampoline_size(m, flags, tlinks, func_addr);
> + if (ret < 0)
> + return ret;

Since arch_bpf_trampoline_size was already called before the trampoline
image was allocated, it seems this call to arch_bpf_trampoline_size is
unnecessary. If this call can be omitted, we can avoid one less dry run.

> +
> + if (ret > ((long)image_end - (long)image))
> + return -EFBIG;
>
> jit_fill_hole(image, (unsigned int)(image_end - image));
> ret = prepare_trampoline(&ctx, im, tlinks, func_addr, nregs, flags); …