Subject: Re: [PATCH bpf-next v6 3/4] bpf, arm64: Impelment bpf_arch_text_poke() for arm64

From: Jean-Philippe Brucker <[email-masked]>
Date: Thu, 7 Jul 2022 17:41:07 +0100

On Sat, Jun 25, 2022 at 12:12:54PM -0400, Xu Kuohai wrote:
> Impelment bpf_arch_text_poke() for arm64, so bpf prog or bpf trampoline

Implement

> diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
> index f08a4447d363..e0e9c705a2e4 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>

nit: keep sorted

>  #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"
> @@ -78,6 +80,15 @@ struct jit_ctx {
>  	int fpb_offset;
>  };
>  
> +struct bpf_plt {
> +	u32 insn_ldr; /* load target */
> +	u32 insn_br;  /* branch to target */
> +	u64 target;   /* target value */
> +} __packed;

don't need __packed

> +
> +#define PLT_TARGET_SIZE   sizeof_field(struct bpf_plt, target)
> +#define PLT_TARGET_OFFSET offsetof(struct bpf_plt, target)
> +
>  static inline void emit(const u32 insn, struct jit_ctx *ctx)
>  {
>  	if (ctx->image != NULL)
> @@ -140,6 +151,12 @@ static inline void emit_a64_mov_i64(const int reg, const u64 val,
>  	}
>  }
>  
> +static inline void emit_bti(u32 insn, struct jit_ctx *ctx)
> +{
> +	if (IS_ENABLED(CONFIG_ARM64_BTI_KERNEL))
> +		emit(insn, ctx);
> +}
> +
>  /*
>   * Kernel addresses in the vmalloc space use at most 48 bits, and the
>   * remaining bits are guaranteed to be 0x1. So we can compose the address
> @@ -235,13 +252,30 @@ static bool is_lsi_offset(int offset, int scale)
>  	return true;
>  }
>  
> +/* generated prologue:
> + *      bti c // if CONFIG_ARM64_BTI_KERNEL
> + *      mov x9, lr
> + *      nop  // POKE_OFFSET
> + *      paciasp // if CONFIG_ARM64_PTR_AUTH_KERNEL

Any reason for the change regarding BTI and pointer auth?  We used to put
'bti c' at the function entry if (BTI && !PA), or 'paciasp' if (BTI && PA),
because 'paciasp' is an implicit BTI.

> + *      stp x29, lr, [sp, #-16]!
> + *      mov x29, sp
> + *      stp x19, x20, [sp, #-16]!
> + *      stp x21, x22, [sp, #-16]!
> + *      stp x25, x26, [sp, #-16]!
> + *      stp x27, x28, [sp, #-16]!
> + *      mov x25, sp
> + *      mov tcc, #0
> + *      // PROLOGUE_OFFSET
> + */
> +
> +#define BTI_INSNS (IS_ENABLED(CONFIG_ARM64_BTI_KERNEL) ? 1 : 0)
> +#define PAC_INSNS (IS_ENABLED(CONFIG_ARM64_PTR_AUTH_KERNEL) ? 1 : 0)
> +
> +/* Offset of nop instruction in bpf prog entry to be poked */
> +#define POKE_OFFSET (BTI_INSNS + 1)
> +
>  /* 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)
>  
>  static int build_prologue(struct jit_ctx *ctx, bool ebpf_from_cbpf)
>  {
> @@ -280,12 +314,14 @@ static int build_prologue(struct jit_ctx *ctx, bool ebpf_from_cbpf)
>  	 *
>  	 */
>  
> +	emit_bti(A64_BTI_C, ctx);
> +
> +	emit(A64_MOV(1, A64_R(9), A64_LR), ctx);
> +	emit(A64_NOP, ctx);
> +
>  	/* 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);
> @@ -312,8 +348,7 @@ static int build_prologue(struct jit_ctx *ctx, bool ebpf_from_cbpf)
>  		}
>  
>  		/* BTI landing pad for the tail call, done with a BR */
> -		if (IS_ENABLED(CONFIG_ARM64_BTI_KERNEL))
> -			emit(A64_BTI_J, ctx);
> +		emit_bti(A64_BTI_J, ctx);
>  	}
>  
>  	emit(A64_SUB_I(1, fpb, fp, ctx->fpb_offset), ctx);
> @@ -557,6 +592,53 @@ static int emit_ll_sc_atomic(const struct bpf_insn *insn, struct jit_ctx *ctx)
>  	return 0;
>  }
>  
> +void dummy_tramp(void);
> +
> +asm (
> +"	.pushsection .text, \"ax\", @progbits\n"
> +"	.type dummy_tramp, %function\n"
> +"dummy_tramp:"
> +#if IS_ENABLED(CONFIG_ARM64_BTI_KERNEL)
> +"	bti j\n" /* dummy_tramp is called via "br x10" */
> +#endif
> +"	mov x10, lr\n"
> +"	mov lr, x9\n"
> +"	ret x10\n"
> +"	.size dummy_tramp, .-dummy_tramp\n"
> +"	.popsection\n"
> +);
> +
> +/* build a plt initialized like this:
> + *
> + * plt:
> + *      ldr tmp, target
> + *      br tmp
> + * target:
> + *      .quad dummy_tramp
> + *
> + * when a long jump trampoline is attached, target is filled with the
> + * trampoline address, and when the trampoine is removed, target is

s/trampoine/trampoline/

> + * restored to dummy_tramp address.
> + */
> +static void build_plt(struct jit_ctx *ctx, bool write_target)
> +{
> +	const u8 tmp = bpf2a64[TMP_REG_1];
> +	struct bpf_plt *plt = NULL;
> +
> +	/* make sure target is 64-bit aligend */

aligned

> +	if ((ctx->idx + PLT_TARGET_OFFSET / AARCH64_INSN_SIZE) % 2)
> +		emit(A64_NOP, ctx);
> +
> +	plt = (struct bpf_plt *)(ctx->image + ctx->idx);
> +	/* plt is called via bl, no BTI needed here */
> +	emit(A64_LDR64LIT(tmp, 2 * AARCH64_INSN_SIZE), ctx);
> +	emit(A64_BR(tmp), ctx);
> +
> +	/* false write_target means target space is not allocated yet */
> +	if (write_target)

How about "if (ctx->image)", to be consistent

> +		plt->target = (u64)&dummy_tramp;
> +}
> +
>  static void build_epilogue(struct jit_ctx *ctx)
>  {
>  	const u8 r0 = bpf2a64[BPF_REG_0];
> @@ -1356,7 +1438,7 @@ struct arm64_jit_data {
>  
>  struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
>  {
> -	int image_size, prog_size, extable_size;
> +	int image_size, prog_size, extable_size, extable_align, extable_offset;
>  	struct bpf_prog *tmp, *orig_prog = prog;
>  	struct bpf_binary_header *header;
>  	struct arm64_jit_data *jit_data;
> @@ -1426,13 +1508,17 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
>  
>  	ctx.epilogue_offset = ctx.idx;
>  	build_epilogue(&ctx);
> +	build_plt(&ctx, false);
>  
> +	extable_align = __alignof__(struct exception_table_entry);
>  	extable_size = prog->aux->num_exentries *
>  		sizeof(struct exception_table_entry);
>  
>  	/* Now we know the actual image size. */
>  	prog_size = sizeof(u32) * ctx.idx;
> -	image_size = prog_size + extable_size;
> +	/* also allocate space for plt target */
> +	extable_offset = round_up(prog_size + PLT_TARGET_SIZE, extable_align);
> +	image_size = extable_offset + extable_size;
>  	header = bpf_jit_binary_alloc(image_size, &image_ptr,
>  				      sizeof(u32), jit_fill_hole);
>  	if (header == NULL) {
> @@ -1444,7 +1530,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
>  
>  	ctx.image = (__le32 *)image_ptr;
>  	if (extable_size)
> -		prog->aux->extable = (void *)image_ptr + prog_size;
> +		prog->aux->extable = (void *)image_ptr + extable_offset;
>  skip_init_ctx:
>  	ctx.idx = 0;
>  	ctx.exentry_idx = 0;
> @@ -1458,6 +1544,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
>  	}
>  
>  	build_epilogue(&ctx);
> +	build_plt(&ctx, true);
>  
>  	/* 3. Extra pass to validate JITed code. */
>  	if (validate_code(&ctx)) {
> @@ -1537,3 +1624,218 @@ bool bpf_jit_supports_subprog_tailcalls(void)
>  {
>  	return true;
>  }
> +
> +static bool is_long_jump(void *ip, void *target)
> +{
> +	long offset;
> +
> +	/* NULL target means this is a NOP */
> +	if (!target)
> +		return false;
> +
> +	offset = (long)target - (long)ip;
> +	return offset < -SZ_128M || offset >= SZ_128M;
> +}
> +
> +static int gen_branch_or_nop(enum aarch64_insn_branch_type type, void *ip,
> +			     void *addr, void *plt, u32 *insn)
> +{
> +	void *target;
> +
> +	if (!addr) {
> +		*insn = aarch64_insn_gen_nop();
> +		return 0;
> +	}
> +
> +	if (is_long_jump(ip, addr))
> +		target = plt;
> +	else
> +		target = addr;
> +
> +	*insn = aarch64_insn_gen_branch_imm((unsigned long)ip,
> +					    (unsigned long)target,
> +					    type);
> +
> +	return *insn != AARCH64_BREAK_FAULT ? 0 : -EFAULT;
> +}
> +
> +/* Replace the branch instruction from @ip to @old_addr in a bpf prog or a bpf
> + * trampoline with the branch instruction from @ip to @new_addr. If @old_addr
> + * or @new_addr is NULL, the old or new instruction is NOP.
> + *
> + * When @ip is the bpf prog entry, a bpf trampoline is being attached or
> + * detached. Since bpf trampoline and bpf prog are allocated separately with
> + * vmalloc, the address distance may exceed 128MB, the maximum branch range.
> + * So long jump should be handled.
> + *
> + * When a bpf prog is constructed, a plt pointing to empty trampoline
> + * dummy_tramp is placed at the end:
> + *
> + *      bpf_prog:
> + *              mov x9, lr
> + *              nop // patchsite
> + *              ...
> + *              ret
> + *
> + *      plt:
> + *              ldr x10, target
> + *              br x10
> + *      target:
> + *              .quad dummy_tramp // plt target
> + *
> + * This is also the state when no trampoline is attached.
> + *
> + * When a short-jump bpf trampoline is attached, the patchsite is patched
> + * to a bl instruction to the trampoline directly:
> + *
> + *      bpf_prog:
> + *              mov x9, lr
> + *              bl <short-jump bpf trampoline address> // patchsite
> + *              ...
> + *              ret
> + *
> + *      plt:
> + *              ldr x10, target
> + *              br x10
> + *      target:
> + *              .quad dummy_tramp // plt target
> + *
> + * When a long-jump bpf trampoline is attached, the plt target is filled with
> + * the trampoline address and the patchsite is patched to a bl instruction to
> + * the plt:
> + *
> + *      bpf_prog:
> + *              mov x9, lr
> + *              bl plt // patchsite
> + *              ...
> + *              ret
> + *
> + *      plt:
> + *              ldr x10, target
> + *              br x10
> + *      target:
> + *              .quad <long-jump bpf trampoline address> // plt target
> + *
> + * The dummy_tramp is used to prevent another CPU from jumping to unknown
> + * locations during the patching process, making the patching process easier.
> + */
> +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;
> +	struct bpf_plt *plt = NULL;
> +	unsigned long size = 0UL;
> +	unsigned long offset = ~0UL;
> +	enum aarch64_insn_branch_type branch_type;
> +	char namebuf[KSYM_NAME_LEN];
> +	void *image = NULL;
> +	u64 plt_target = 0ULL;
> +	bool poking_bpf_entry;
> +
> +	if (!__bpf_address_lookup((unsigned long)ip, &size, &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 -ENOTSUPP;
> +
> +	image = ip - offset;
> +	/* zero offset means we're poking bpf prog entry */
> +	poking_bpf_entry = (offset == 0UL);
> +
> +	/* bpf prog entry, find plt and the real patchsite */
> +	if (poking_bpf_entry) {
> +		/* plt locates at the end of bpf prog */
> +		plt = image + size - PLT_TARGET_OFFSET;
> +
> +		/* skip to the nop instruction in bpf prog entry:
> +		 * bti c // if BTI enabled
> +		 * mov x9, x30
> +		 * nop
> +		 */
> +		ip = image + POKE_OFFSET * AARCH64_INSN_SIZE;
> +	}
> +
> +	/* long jump is only possible at bpf prog entry */
> +	if (WARN_ON((is_long_jump(ip, new_addr) || is_long_jump(ip, old_addr)) &&
> +		    !poking_bpf_entry))
> +		return -EINVAL;
> +
> +	if (poke_type == BPF_MOD_CALL)
> +		branch_type = AARCH64_INSN_BRANCH_LINK;
> +	else
> +		branch_type = AARCH64_INSN_BRANCH_NOLINK;
> +
> +	if (gen_branch_or_nop(branch_type, ip, old_addr, plt, &old_insn) < 0)
> +		return -EFAULT;
> +
> +	if (gen_branch_or_nop(branch_type, ip, new_addr, plt, &new_insn) < 0)
> +		return -EFAULT;
> +
> +	if (is_long_jump(ip, new_addr))
> +		plt_target = (u64)new_addr;
> +	else if (is_long_jump(ip, old_addr))
> +		/* if the old target is a long jump and the new target is not,
> +		 * restore the plt target to dummy_tramp, so there is always a
> +		 * legal and harmless address stored in plt target, and we'll
> +		 * never jump from plt to an unknown place.
> +		 */
> +		plt_target = (u64)&dummy_tramp;
> +
> +	if (plt_target) {
> +		/* non-zero plt_target indicates we're patching a bpf prog,
> +		 * which is read only.
> +		 */
> +		if (set_memory_rw(PAGE_MASK & ((uintptr_t)&plt->target), 1))
> +			return -EFAULT;
> +		WRITE_ONCE(plt->target, plt_target);
> +		set_memory_ro(PAGE_MASK & ((uintptr_t)&plt->target), 1);
> +		/* since plt target points to either the new trmapoline

trampoline

> +		 * or dummy_tramp, even if aother CPU reads the old plt

another

Thanks,
Jean

> +		 * target value before fetching the bl instruction to plt,
> +		 * it will be brought back by dummy_tramp, so no barrier is
> +		 * required here.
> +		 */
> +	}
> +
> +	/* if the old target and the new target are both long jumps, no
> +	 * patching is required
> +	 */
> +	if (old_insn == new_insn)
> +		return 0;
> +
> +	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 executes the
> +	 * old instruction after the patching operation finishes (e.g.,
> +	 * pipeline not flushed, or icache not synchronized yet).
> +	 *
> +	 * 1. when a new trampoline is attached, it is not a problem 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 task rcu to do the sync, no need to call
> +	 *    the sync version here, see bpf_tramp_image_put() for details.
> +	 */
> +	ret = aarch64_insn_patch_text_nosync(ip, new_insn);
> +out:
> +	mutex_unlock(&text_mutex);
> +
> +	return ret;
> +}
> -- 
> 2.30.2
>