Re: [PATCH RESEND bpf 2/2] x86/bpf: Fix IP for relocating call depth accounting

From: Alexei Starovoitov
Date: Fri Mar 29 2024 - 17:53:32 EST

On Fri, Mar 29, 2024 at 2:49 AM Uros Bizjak <ubizjak@xxxxxxxxx> wrote:
>
> From: Joan Bruguera Micó <joanbrugueram@xxxxxxxxx>
>
> The recently introduced support for %rip-relative relocations in the
> call thunk template assumes that the code is being patched in-place,
> so the destination of the relocation matches the address of the code.
> This is not true for the call depth accounting emitted by the BPF JIT,
> so the calculated address is wrong and usually causes a page fault.

Could you share the link to what this 'rip-relative' relocation is ?

> Pass the destination IP when the BPF JIT emits call depth accounting.
>
> Fixes: 17bce3b2ae2d ("x86/callthunks: Handle %rip-relative relocations in call thunk template")

Ohh. It's buried inside that patch.
Pls make commit log a bit more clear that that commit 17bce3b2ae2d
broke x86_call_depth_emit_accounting logic.

> Signed-off-by: Joan Bruguera Micó <joanbrugueram@xxxxxxxxx>
> Reviewed-by: Uros Bizjak <ubizjak@xxxxxxxxx>
> Acked-by: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
> Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> ---
> arch/x86/include/asm/alternative.h | 4 ++--
> arch/x86/kernel/callthunks.c | 4 ++--
> arch/x86/net/bpf_jit_comp.c | 19 ++++++++-----------
> 3 files changed, 12 insertions(+), 15 deletions(-)
>
> diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h
> index fcd20c6dc7f9..67b68d0d17d1 100644
> --- a/arch/x86/include/asm/alternative.h
> +++ b/arch/x86/include/asm/alternative.h
> @@ -117,7 +117,7 @@ extern void callthunks_patch_builtin_calls(void);
> extern void callthunks_patch_module_calls(struct callthunk_sites *sites,
> struct module *mod);
> extern void *callthunks_translate_call_dest(void *dest);
> -extern int x86_call_depth_emit_accounting(u8 **pprog, void *func);
> +extern int x86_call_depth_emit_accounting(u8 **pprog, void *func, void *ip);
> #else
> static __always_inline void callthunks_patch_builtin_calls(void) {}
> static __always_inline void
> @@ -128,7 +128,7 @@ static __always_inline void *callthunks_translate_call_dest(void *dest)
> return dest;
> }
> static __always_inline int x86_call_depth_emit_accounting(u8 **pprog,
> - void *func)
> + void *func, void *ip)
> {
> return 0;
> }
> diff --git a/arch/x86/kernel/callthunks.c b/arch/x86/kernel/callthunks.c
> index 30335182b6b0..e92ff0c11db8 100644
> --- a/arch/x86/kernel/callthunks.c
> +++ b/arch/x86/kernel/callthunks.c
> @@ -314,7 +314,7 @@ static bool is_callthunk(void *addr)
> return !bcmp(pad, insn_buff, tmpl_size);
> }
>
> -int x86_call_depth_emit_accounting(u8 **pprog, void *func)
> +int x86_call_depth_emit_accounting(u8 **pprog, void *func, void *ip)
> {
> unsigned int tmpl_size = SKL_TMPL_SIZE;
> u8 insn_buff[MAX_PATCH_LEN];
> @@ -327,7 +327,7 @@ int x86_call_depth_emit_accounting(u8 **pprog, void *func)
> return 0;
>
> memcpy(insn_buff, skl_call_thunk_template, tmpl_size);
> - apply_relocation(insn_buff, tmpl_size, *pprog,
> + apply_relocation(insn_buff, tmpl_size, ip,

Did the logic inside apply_relocation() change to have
a new meaning for 'dest' and 'src'?
Answering to myself... yes. in that commit.
Better commit log would have made the code review so much easier.

> skl_call_thunk_template, tmpl_size);
>
> memcpy(*pprog, insn_buff, tmpl_size);
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index 09f7dc9d4d65..f2e8769f5eee 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -481,7 +481,7 @@ static int emit_rsb_call(u8 **pprog, void *func, void *ip)
> {
> void *adjusted_ip;
> OPTIMIZER_HIDE_VAR(func);
> - adjusted_ip = ip + x86_call_depth_emit_accounting(pprog, func);
> + adjusted_ip = ip + x86_call_depth_emit_accounting(pprog, func, ip);

Now I see why you added extra var in the previous patch.
Should have mentioned it in the commit log.

> return emit_patch(pprog, func, adjusted_ip, 0xE8);
> }
>
> @@ -1973,20 +1973,17 @@ st: if (is_imm8(insn->off))
>
> /* call */
> case BPF_JMP | BPF_CALL: {
> - int offs;
> + u8 *ip = image + addrs[i - 1];
>
> func = (u8 *) __bpf_call_base + imm32;
> if (tail_call_reachable) {
> RESTORE_TAIL_CALL_CNT(bpf_prog->aux->stack_depth);
> - if (!imm32)
> - return -EINVAL;
> - offs = 7 + x86_call_depth_emit_accounting(&prog, func);
> - } else {
> - if (!imm32)
> - return -EINVAL;
> - offs = x86_call_depth_emit_accounting(&prog, func);
> + ip += 7;
> }
> - if (emit_call(&prog, func, image + addrs[i - 1] + offs))
> + if (!imm32)
> + return -EINVAL;
> + ip += x86_call_depth_emit_accounting(&prog, func, ip);
> + if (emit_call(&prog, func, ip))
> return -EINVAL;
> break;
> }
> @@ -2836,7 +2833,7 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *rw_im
> * Direct-call fentry stub, as such it needs accounting for the
> * __fentry__ call.
> */
> - x86_call_depth_emit_accounting(&prog, NULL);
> + x86_call_depth_emit_accounting(&prog, NULL, image);

Overall it all makes sense.
Pls respin with more precise commit logs.