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From: Uros Bizjak <ubizjak@gmail.com>
Date: Sat, 30 Mar 2024 10:01:04 +0100
Message-ID: <CAFULd4b6juiw3wC3Z61V9=-UnA+NGyUt4231vC14UnGAATk6tA@mail.gmail.com>
Subject: Re: [PATCH RESEND bpf 2/2] x86/bpf: Fix IP for relocating call depth accounting
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: X86 ML <x86@kernel.org>, bpf <bpf@vger.kernel.org>, 
	Network Development <netdev@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>, 
	=?UTF-8?Q?Joan_Bruguera_Mic=C3=B3?= <joanbrugueram@gmail.com>, 
	Ingo Molnar <mingo@kernel.org>, Alexei Starovoitov <ast@kernel.org>, 
	Daniel Borkmann <daniel@iogearbox.net>
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On Fri, Mar 29, 2024 at 10:53=E2=80=AFPM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> On Fri, Mar 29, 2024 at 2:49=E2=80=AFAM Uros Bizjak <ubizjak@gmail.com> w=
rote:
> >
> > From: Joan Bruguera Mic=C3=B3 <joanbrugueram@gmail.com>
> >
> > 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 ?

Please see the "RIP relative addressing" section in [1].

[1] https://compas.cs.stonybrook.edu/~nhonarmand/courses/sp17/cse506/ref/as=
sembly.html

In our case:

The callthunks patching creates a call thunk template in the .rodata
section (please see arch/x86/kernel/callthunks.c)  that is later
copied to the .text section at the correct place. The template uses
X86_call_depth in the pcpu_hot structure. Previously, the template
used absolute location for X86_call_depth and the linker resolved the
address in the template to this absolute location. There is no issue
when this template is copied to the various places in the .text
section.

When we want to use PC relative relocations (to reduce the code size),
then the linker calculates the address of the variable in the template
according to the PC in the .rodata section. If we want to copy the
template to its final location, then the address of X86_call_depth,
relative to the PC, has to be adjusted, as explained in
arch/x86/kernel/alternative.c, in the comment above apply_reloc_n
macro.

Uros.

> > 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 Mic=C3=B3 <joanbrugueram@gmail.com>
> > Reviewed-by: Uros Bizjak <ubizjak@gmail.com>
> > Acked-by: Ingo Molnar <mingo@kernel.org>
> > Cc: Alexei Starovoitov <ast@kernel.org>
> > Cc: Daniel Borkmann <daniel@iogearbox.net>
> > ---
> >  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 *site=
s,
> >                                           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_c=
all_dest(void *dest)
> >         return dest;
> >  }
> >  static __always_inline int x86_call_depth_emit_accounting(u8 **pprog,
> > -                                                         void *func)
> > +                                                         void *func, v=
oid *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 =3D 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, vo=
id *ip)
> >  {
> >         void *adjusted_ip;
> >         OPTIMIZER_HIDE_VAR(func);
> > -       adjusted_ip =3D ip + x86_call_depth_emit_accounting(pprog, func=
);
> > +       adjusted_ip =3D 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 =3D image + addrs[i - 1];
> >
> >                         func =3D (u8 *) __bpf_call_base + imm32;
> >                         if (tail_call_reachable) {
> >                                 RESTORE_TAIL_CALL_CNT(bpf_prog->aux->st=
ack_depth);
> > -                               if (!imm32)
> > -                                       return -EINVAL;
> > -                               offs =3D 7 + x86_call_depth_emit_accoun=
ting(&prog, func);
> > -                       } else {
> > -                               if (!imm32)
> > -                                       return -EINVAL;
> > -                               offs =3D x86_call_depth_emit_accounting=
(&prog, func);
> > +                               ip +=3D 7;
> >                         }
> > -                       if (emit_call(&prog, func, image + addrs[i - 1]=
 + offs))
> > +                       if (!imm32)
> > +                               return -EINVAL;
> > +                       ip +=3D x86_call_depth_emit_accounting(&prog, f=
unc, ip);
> > +                       if (emit_call(&prog, func, ip))
> >                                 return -EINVAL;
> >                         break;
> >                 }
> > @@ -2836,7 +2833,7 @@ static int __arch_prepare_bpf_trampoline(struct b=
pf_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.