Message-ID: <Yo9X0EKU03QbFZk+@FVFF77S0Q05N>
Date:   Thu, 26 May 2022 11:34:56 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Xu Kuohai <xukuohai@...wei.com>
Cc:     bpf@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
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        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
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        Ingo Molnar <mingo@...hat.com>,
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        Zi Shen Lim <zlim.lnx@...il.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        "David S . Miller" <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
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        Mark Brown <broonie@...nel.org>,
        Delyan Kratunov <delyank@...com>,
        Kumar Kartikeya Dwivedi <memxor@...il.com>
Subject: Re: [PATCH bpf-next v5 4/6] bpf, arm64: Impelment
 bpf_arch_text_poke() for arm64

On Thu, May 26, 2022 at 05:45:30PM +0800, Xu Kuohai wrote:
> On 5/25/2022 10:10 PM, Mark Rutland wrote:
> > On Wed, May 18, 2022 at 09:16:36AM -0400, Xu Kuohai wrote:
> >> Impelment bpf_arch_text_poke() for arm64, so bpf trampoline code can use
> >> it to replace nop with jump, or replace jump with nop.
> >>
> >> Signed-off-by: Xu Kuohai <xukuohai@...wei.com>
> >> Acked-by: Song Liu <songliubraving@...com>
> >> Reviewed-by: Jakub Sitnicki <jakub@...udflare.com>
> >> ---
> >>  arch/arm64/net/bpf_jit.h      |   1 +
> >>  arch/arm64/net/bpf_jit_comp.c | 107 +++++++++++++++++++++++++++++++---
> >>  2 files changed, 99 insertions(+), 9 deletions(-)
> >>
> >> diff --git a/arch/arm64/net/bpf_jit.h b/arch/arm64/net/bpf_jit.h
> >> index 194c95ccc1cf..1c4b0075a3e2 100644
> >> --- a/arch/arm64/net/bpf_jit.h
> >> +++ b/arch/arm64/net/bpf_jit.h
> >> @@ -270,6 +270,7 @@
> >>  #define A64_BTI_C  A64_HINT(AARCH64_INSN_HINT_BTIC)
> >>  #define A64_BTI_J  A64_HINT(AARCH64_INSN_HINT_BTIJ)
> >>  #define A64_BTI_JC A64_HINT(AARCH64_INSN_HINT_BTIJC)
> >> +#define A64_NOP    A64_HINT(AARCH64_INSN_HINT_NOP)
> >>  
> >>  /* DMB */
> >>  #define A64_DMB_ISH aarch64_insn_gen_dmb(AARCH64_INSN_MB_ISH)
> >> diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
> >> index 8ab4035dea27..5ce6ed5f42a1 100644
> >> --- a/arch/arm64/net/bpf_jit_comp.c
> >> +++ b/arch/arm64/net/bpf_jit_comp.c
> >> @@ -9,6 +9,7 @@
> >>  
> >>  #include <linux/bitfield.h>
> >>  #include <linux/bpf.h>
> >> +#include <linux/memory.h>
> >>  #include <linux/filter.h>
> >>  #include <linux/printk.h>
> >>  #include <linux/slab.h>
> >> @@ -18,6 +19,7 @@
> >>  #include <asm/cacheflush.h>
> >>  #include <asm/debug-monitors.h>
> >>  #include <asm/insn.h>
> >> +#include <asm/patching.h>
> >>  #include <asm/set_memory.h>
> >>  
> >>  #include "bpf_jit.h"
> >> @@ -235,13 +237,13 @@ static bool is_lsi_offset(int offset, int scale)
> >>  	return true;
> >>  }
> >>  
> >> +#define BTI_INSNS (IS_ENABLED(CONFIG_ARM64_BTI_KERNEL) ? 1 : 0)
> >> +#define PAC_INSNS (IS_ENABLED(CONFIG_ARM64_PTR_AUTH_KERNEL) ? 1 : 0)
> >> +
> >>  /* Tail call offset to jump into */
> >> -#if IS_ENABLED(CONFIG_ARM64_BTI_KERNEL) || \
> >> -	IS_ENABLED(CONFIG_ARM64_PTR_AUTH_KERNEL)
> >> -#define PROLOGUE_OFFSET 9
> >> -#else
> >> -#define PROLOGUE_OFFSET 8
> >> -#endif
> >> +#define PROLOGUE_OFFSET	(BTI_INSNS + 2 + PAC_INSNS + 8)
> >> +/* Offset of nop instruction in bpf prog entry to be poked */
> >> +#define POKE_OFFSET	(BTI_INSNS + 1)
> >>  
> >>  static int build_prologue(struct jit_ctx *ctx, bool ebpf_from_cbpf)
> >>  {
> >> @@ -279,12 +281,15 @@ static int build_prologue(struct jit_ctx *ctx, bool ebpf_from_cbpf)
> >>  	 *
> >>  	 */
> >>  
> >> +	if (IS_ENABLED(CONFIG_ARM64_BTI_KERNEL))
> >> +		emit(A64_BTI_C, ctx);
> >> +
> >> +	emit(A64_MOV(1, A64_R(9), A64_LR), ctx);
> >> +	emit(A64_NOP, ctx);
> > 
> > I take it the idea is to make this the same as the regular ftrace patch-site
> > sequence, so that this can call the same trampoline(s) ?
> > 
> 
> Yes, we can attach a bpf trampoline to bpf prog.

Just to check, is the BPF trampoline *only* attached to BPF programs, or could
that be attached to a regular ftrace patch-site?

I has assumed that the point of adding direct calls support was so that this
could be called from regular ftrace patch sites, but your replies below on how
the trampoline is protected imply that's not the case.

> > If so, we need some commentary to that effect, and we need some comments in the
> > ftrace code explaining that this needs to be kept in-sync.
> > 
> 
> This is patched by bpf_arch_text_poke(), not ftrace.

I understood that, but if the idea is that the instruction sequence must match,
then we need to make that clear. Otherwise changes to one side may break the
other unexpectedly.

> >> +
> >>  	/* Sign lr */
> >>  	if (IS_ENABLED(CONFIG_ARM64_PTR_AUTH_KERNEL))
> >>  		emit(A64_PACIASP, ctx);
> >> -	/* BTI landing pad */
> >> -	else if (IS_ENABLED(CONFIG_ARM64_BTI_KERNEL))
> >> -		emit(A64_BTI_C, ctx);
> >>  
> >>  	/* Save FP and LR registers to stay align with ARM64 AAPCS */
> >>  	emit(A64_PUSH(A64_FP, A64_LR, A64_SP), ctx);
> >> @@ -1529,3 +1534,87 @@ void bpf_jit_free_exec(void *addr)
> >>  {
> >>  	return vfree(addr);
> >>  }
> >> +
> >> +static int gen_branch_or_nop(enum aarch64_insn_branch_type type, void *ip,
> >> +			     void *addr, u32 *insn)
> >> +{
> >> +	if (!addr)
> >> +		*insn = aarch64_insn_gen_nop();
> >> +	else
> >> +		*insn = aarch64_insn_gen_branch_imm((unsigned long)ip,
> >> +						    (unsigned long)addr,
> >> +						    type);
> >> +
> >> +	return *insn != AARCH64_BREAK_FAULT ? 0 : -EFAULT;
> >> +}
> >> +
> >> +int bpf_arch_text_poke(void *ip, enum bpf_text_poke_type poke_type,
> >> +		       void *old_addr, void *new_addr)
> >> +{
> >> +	int ret;
> >> +	u32 old_insn;
> >> +	u32 new_insn;
> >> +	u32 replaced;
> >> +	unsigned long offset = ~0UL;
> >> +	enum aarch64_insn_branch_type branch_type;
> >> +	char namebuf[KSYM_NAME_LEN];
> >> +
> >> +	if (!__bpf_address_lookup((unsigned long)ip, NULL, &offset, namebuf))
> >> +		/* Only poking bpf text is supported. Since kernel function
> >> +		 * entry is set up by ftrace, we reply on ftrace to poke kernel
> >> +		 * functions.
> >> +		 */
> >> +		return -EINVAL;
> >> +
> >> +	/* bpf entry */
> >> +	if (offset == 0UL)
> >> +		/* skip to the nop instruction in bpf prog entry:
> >> +		 * bti c	// if BTI enabled
> >> +		 * mov x9, x30
> >> +		 * nop
> >> +		 */
> >> +		ip = ip + POKE_OFFSET * AARCH64_INSN_SIZE;
> > 
> > When is offset non-zero? is this ever called to patch other instructions, and
> > could this ever be used to try to patch the BTI specifically?
> 
> bpf_arch_text_poke() is also called to patch other instructions, for
> example, bpf_tramp_image_put() calls this to skip calling fexit bpf progs:
> 
> int err = bpf_arch_text_poke(im->ip_after_call, BPF_MOD_JUMP,
>                              NULL, im->ip_epilogue);
> 
> 
> Before this is called, a bpf trampoline looks like:
> 
> 	[...]
> ip_after_call:
> 	nop // to be patched
> 	bl <fexit prog>
> ip_epilogue:
> 	bti j
> 	[...]
> 
> After:
> 	[...]
> ip_after_call:
> 	b <ip_epilogue> // patched
> 	bl <fexit prog>
> ip_epilogue:
> 	bti j
> 	[...]
> 
> 
> > I strongly suspect we need a higher-level API to say "poke the patchable
> > callsite in the prologue", rather than assuming that offset 0 always means
> > that, or it'll be *very* easy for this to go wrong.
> 
> Ah, bpf_arch_text_poke() only patches bpf progs, and the patch-site in
> bpf prog prologue is constructed for bpf_arch_text_poke(), so we always
> know the patch-site offset. There's no compiler generated instruction
> here, so it seems to be not a problem.

I understand all of that. My point is that if anyone ever wants to use
bpf_arch_text_poke() to poke the *actual* first instruction, this will go
wrong.

I see that x86 does the same thing to skip ENDBR, so if this is just a fragile
API, and people are expected to not do that, then fine. It's just confusing.

> >> +
> >> +	if (poke_type == BPF_MOD_CALL)
> >> +		branch_type = AARCH64_INSN_BRANCH_LINK;
> >> +	else
> >> +		branch_type = AARCH64_INSN_BRANCH_NOLINK;
> > 
> > When is poke_type *not* BPF_MOD_CALL?>
> 
> The bpf_tramp_image_put() example above uses BPF_MOD_JUMP.
> 
> > I assume that means BPF also uses this for non-ftrace reasons?
> >
> 
> This function is NOT used for ftrace patch-site. It's only used to patch
> bpf image.

I understand that this isn't ftrace; I meant for the patch site we introduced
in the BPF program that is compatible with the patch-site sequence that ftrace
uses.

> >> +	if (gen_branch_or_nop(branch_type, ip, old_addr, &old_insn) < 0)
> >> +		return -EFAULT;
> >> +
> >> +	if (gen_branch_or_nop(branch_type, ip, new_addr, &new_insn) < 0)
> >> +		return -EFAULT;
> >> +
> >> +	mutex_lock(&text_mutex);
> >> +	if (aarch64_insn_read(ip, &replaced)) {
> >> +		ret = -EFAULT;
> >> +		goto out;
> >> +	}
> >> +
> >> +	if (replaced != old_insn) {
> >> +		ret = -EFAULT;
> >> +		goto out;
> >> +	}
> >> +
> >> +	/* We call aarch64_insn_patch_text_nosync() to replace instruction
> >> +	 * atomically, so no other CPUs will fetch a half-new and half-old
> >> +	 * instruction. But there is chance that another CPU fetches the old
> >> +	 * instruction after bpf_arch_text_poke() finishes, that is, different
> >> +	 * CPUs may execute different versions of instructions at the same
> >> +	 * time before the icache is synchronized by hardware.
> >> +	 *
> >> +	 * 1. when a new trampoline is attached, it is not an issue for
> >> +	 *    different CPUs to jump to different trampolines temporarily.
> >> +	 *
> >> +	 * 2. when an old trampoline is freed, we should wait for all other
> >> +	 *    CPUs to exit the trampoline and make sure the trampoline is no
> >> +	 *    longer reachable, since bpf_tramp_image_put() function already
> >> +	 *    uses percpu_ref and rcu task to do the sync, no need to call the
> >> +	 *    sync interface here.
> >> +	 */
> > 
> > How is RCU used for that? It's not clear to me how that works for PREEMPT_RCU
> > (which is the usual configuration for arm64), since we can easily be in a
> > preemptible context, outside of an RCU read side critical section, yet call
> > into a trampoline.
> > 
> > I know that for livepatching we need to use stacktracing to ensure we've
> > finished using code we'd like to free, and I can't immediately see how you can
> > avoid that here. I'm suspicious that there's still a race where threads can
> > enter the trampoline and it can be subsequently freed.
> > 
> > For ftrace today we get away with entering the existing trampolines when not
> > intended because those are statically allocated, and the race is caught when
> > acquiring the ops inside the ftrace core code. This case is different because
> > the CPU can fetch the instruction and execute that at any time, without any RCU
> > involvement.
> > 
> > Can you give more details on how the scheme described above works? How
> > *exactly*` do you ensure that threads which have entered the trampoline (and
> > may have been immediately preempted by an interrupt) have returned? Which RCU
> > mechanism are you using?
> > 
> > If you can point me at where this is implemented I'm happy to take a look.
> >
> IIUC, task rcu's critical section ends at a voluntary context switch,
> since no volutary context switch occurs in a progoluge, when a task
> rcu's critical section ends, we can ensure no one is running in the
> prologue [1].
> 
> For bpf trampoline, the scenario is similar, except that it may sleep,
> so a reference count is increased when entering the trampoline and
> decreased when exiting the trampoline, so we can wait for the reference
> count to become zero to make sure there is no one in the sleepable
> region [2].
> 
> [1] https://lore.kernel.org/all/20140804192017.GA18337@linux.vnet.ibm.com/
> [2]
> https://lore.kernel.org/bpf/20210316210007.38949-1-alexei.starovoitov@gmail.com/

Ok. Am I correct in understanding that this means the BPF trampoline is only
ever attached to BPF programs and those programs handle this around calling the
BPF trampoline?

Thanks,
Mark.

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