[PATCH bpf-next v6 3/4] bpf, arm64: Impelment bpf_arch_text_poke() for arm64
Jean-Philippe Brucker
jean-philippe at linaro.org
Thu Jul 7 09:41:07 PDT 2022
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
>
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