Re: [PATCH bpf] riscv, bpf: Adapt bpf trampoline to optimized riscv ftrace framework
From: Pu Lehui
Date: Wed Jul 19 2023 - 10:45:27 EST
On 2023/7/19 4:06, Björn Töpel wrote:
Pu Lehui <pulehui@xxxxxxxxxxxxxxx> writes:
From: Pu Lehui <pulehui@xxxxxxxxxx>
Commit 6724a76cff85 ("riscv: ftrace: Reduce the detour code size to
half") optimizes the detour code size of kernel functions to half with
T0 register and the upcoming DYNAMIC_FTRACE_WITH_DIRECT_CALLS of riscv
is based on this optimization, we need to adapt riscv bpf trampoline
based on this. One thing to do is to reduce detour code size of bpf
programs, and the second is to deal with the return address after the
execution of bpf trampoline. Meanwhile, add more comments and rename
some variables to make more sense. The related tests have passed.
This adaptation needs to be merged before the upcoming
DYNAMIC_FTRACE_WITH_DIRECT_CALLS of riscv, otherwise it will crash due
to a mismatch in the return address. So we target this modification to
bpf tree and add fixes tag for locating.
Thank you for working on this!
Fixes: 6724a76cff85 ("riscv: ftrace: Reduce the detour code size to half")
This is not a fix. Nothing is broken. Only that this patch much come
before or as part of the ftrace series.
Yep, it's really not a fix. I have no idea whether this patch target to
bpf-next tree can be ahead of the ftrace series of riscv tree?
>> Signed-off-by: Pu Lehui <pulehui@xxxxxxxxxx>
---
arch/riscv/net/bpf_jit_comp64.c | 110 ++++++++++++++------------------
1 file changed, 47 insertions(+), 63 deletions(-)
diff --git a/arch/riscv/net/bpf_jit_comp64.c b/arch/riscv/net/bpf_jit_comp64.c
index c648864c8cd1..ffc9aa42f918 100644
--- a/arch/riscv/net/bpf_jit_comp64.c
+++ b/arch/riscv/net/bpf_jit_comp64.c
@@ -241,7 +241,7 @@ static void __build_epilogue(bool is_tail_call, struct rv_jit_context *ctx)
if (!is_tail_call)
emit_mv(RV_REG_A0, RV_REG_A5, ctx);
emit_jalr(RV_REG_ZERO, is_tail_call ? RV_REG_T3 : RV_REG_RA,
- is_tail_call ? 20 : 0, /* skip reserved nops and TCC init */
+ is_tail_call ? 12 : 0, /* skip reserved nops and TCC init */
Maybe be explicit, and use the "DETOUR_INSNS" from below (and convert to
bytes)?
ctx);
}
@@ -618,32 +618,7 @@ static int add_exception_handler(const struct bpf_insn *insn,
return 0;
}
-static int gen_call_or_nops(void *target, void *ip, u32 *insns)
-{
- s64 rvoff;
- int i, ret;
- struct rv_jit_context ctx;
-
- ctx.ninsns = 0;
- ctx.insns = (u16 *)insns;
-
- if (!target) {
- for (i = 0; i < 4; i++)
- emit(rv_nop(), &ctx);
- return 0;
- }
-
- rvoff = (s64)(target - (ip + 4));
- emit(rv_sd(RV_REG_SP, -8, RV_REG_RA), &ctx);
- ret = emit_jump_and_link(RV_REG_RA, rvoff, false, &ctx);
- if (ret)
- return ret;
- emit(rv_ld(RV_REG_RA, -8, RV_REG_SP), &ctx);
-
- return 0;
-}
-
-static int gen_jump_or_nops(void *target, void *ip, u32 *insns)
+static int gen_jump_or_nops(void *target, void *ip, u32 *insns, bool is_call)
{
s64 rvoff;
struct rv_jit_context ctx;
@@ -658,38 +633,38 @@ static int gen_jump_or_nops(void *target, void *ip, u32 *insns)
}
rvoff = (s64)(target - ip);
- return emit_jump_and_link(RV_REG_ZERO, rvoff, false, &ctx);
+ return emit_jump_and_link(is_call ? RV_REG_T0 : RV_REG_ZERO,
+ rvoff, false, &ctx);
Nit: Please use the full 100 char width.
}
+#define DETOUR_NINSNS 2
Better name? Maybe call this patchable function entry something? Also,
How about RV_FENTRY_NINSNS?
to catch future breaks like this -- would it make sense to have a
static_assert() combined with something tied to
-fpatchable-function-entry= from arch/riscv/Makefile?
It is very necessary, but it doesn't seem to be easy. I try to find GCC
related functions, something like __builtin_xxx, but I can't find it so
far. Also try to make it as a CONFIG_PATCHABLE_FUNCTION_ENTRY=4 in
Makefile and then static_assert, but obviously it shouldn't be done.
Maybe we can deal with this later when we have a solution?
+
int bpf_arch_text_poke(void *ip, enum bpf_text_poke_type poke_type,
void *old_addr, void *new_addr)
{
- u32 old_insns[4], new_insns[4];
+ u32 old_insns[DETOUR_NINSNS], new_insns[DETOUR_NINSNS];
bool is_call = poke_type == BPF_MOD_CALL;
- int (*gen_insns)(void *target, void *ip, u32 *insns);
- int ninsns = is_call ? 4 : 2;
int ret;
- if (!is_bpf_text_address((unsigned long)ip))
+ if (!is_kernel_text((unsigned long)ip) &&
+ !is_bpf_text_address((unsigned long)ip))
return -ENOTSUPP;
- gen_insns = is_call ? gen_call_or_nops : gen_jump_or_nops;
-
- ret = gen_insns(old_addr, ip, old_insns);
+ ret = gen_jump_or_nops(old_addr, ip, old_insns, is_call);
if (ret)
return ret;
- if (memcmp(ip, old_insns, ninsns * 4))
+ if (memcmp(ip, old_insns, DETOUR_NINSNS * 4))
return -EFAULT;
- ret = gen_insns(new_addr, ip, new_insns);
+ ret = gen_jump_or_nops(new_addr, ip, new_insns, is_call);
if (ret)
return ret;
cpus_read_lock();
mutex_lock(&text_mutex);
- if (memcmp(ip, new_insns, ninsns * 4))
- ret = patch_text(ip, new_insns, ninsns);
+ if (memcmp(ip, new_insns, DETOUR_NINSNS * 4))
+ ret = patch_text(ip, new_insns, DETOUR_NINSNS);
mutex_unlock(&text_mutex);
cpus_read_unlock();
@@ -717,7 +692,7 @@ static void restore_args(int nregs, int args_off, struct rv_jit_context *ctx)
}
static int invoke_bpf_prog(struct bpf_tramp_link *l, int args_off, int retval_off,
- int run_ctx_off, bool save_ret, struct rv_jit_context *ctx)
+ int run_ctx_off, bool save_retval, struct rv_jit_context *ctx)
Why the save_retval name change? This churn is not needed IMO
(especially since you keep using the _ret name below). Please keep the
old name. >
{
int ret, branch_off;
struct bpf_prog *p = l->link.prog;
@@ -757,7 +732,7 @@ static int invoke_bpf_prog(struct bpf_tramp_link *l, int args_off, int retval_of
if (ret)
return ret;
- if (save_ret)
+ if (save_retval)
emit_sd(RV_REG_FP, -retval_off, regmap[BPF_REG_0], ctx);
/* update branch with beqz */
@@ -787,20 +762,19 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im,
int i, ret, offset;
int *branches_off = NULL;
int stack_size = 0, nregs = m->nr_args;
- int retaddr_off, fp_off, retval_off, args_off;
- int nregs_off, ip_off, run_ctx_off, sreg_off;
+ int fp_off, retval_off, args_off, nregs_off, ip_off, run_ctx_off, sreg_off;
struct bpf_tramp_links *fentry = &tlinks[BPF_TRAMP_FENTRY];
struct bpf_tramp_links *fexit = &tlinks[BPF_TRAMP_FEXIT];
struct bpf_tramp_links *fmod_ret = &tlinks[BPF_TRAMP_MODIFY_RETURN];
void *orig_call = func_addr;
- bool save_ret;
+ bool save_retval, traced_ret;
u32 insn;
/* Generated trampoline stack layout:
*
* FP - 8 [ RA of parent func ] return address of parent
* function
- * FP - retaddr_off [ RA of traced func ] return address of traced
+ * FP - 16 [ RA of traced func ] return address of
traced
BPF code uses frame pointers. Shouldn't the trampoline frame look like a
regular frame [1], i.e. start with return address followed by previous
frame pointer?
oops, will fix it. Also we need to consider two types of trampoline
stack layout, that is:
* 1. trampoline called from function entry
* --------------------------------------
* FP + 8 [ RA of parent func ] return address of parent
* function
* FP + 0 [ FP ]
*
* FP - 8 [ RA of traced func ] return address of traced
* function
* FP - 16 [ FP ]
* --------------------------------------
*
* 2. trampoline called directly
* --------------------------------------
* FP - 8 [ RA of caller func ] return address of caller
* function
* FP - 16 [ FP ]
* --------------------------------------
* function
* FP - fp_off [ FP of parent func ]
*
@@ -833,17 +807,20 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im,
if (nregs > 8)
return -ENOTSUPP;
- /* room for parent function return address */
+ /* room for return address of parent function */
stack_size += 8;
- stack_size += 8;
- retaddr_off = stack_size;
+ /* whether return to return address of traced function after bpf trampoline */
+ traced_ret = func_addr && !(flags & BPF_TRAMP_F_SKIP_FRAME);
+ /* room for return address of traced function */
+ if (traced_ret)
+ stack_size += 8;
stack_size += 8;
fp_off = stack_size;
- save_ret = flags & (BPF_TRAMP_F_CALL_ORIG | BPF_TRAMP_F_RET_FENTRY_RET);
- if (save_ret) {
+ save_retval = flags & (BPF_TRAMP_F_CALL_ORIG | BPF_TRAMP_F_RET_FENTRY_RET);
+ if (save_retval) {
stack_size += 8;
retval_off = stack_size;
}
@@ -869,7 +846,11 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im,
emit_addi(RV_REG_SP, RV_REG_SP, -stack_size, ctx);
- emit_sd(RV_REG_SP, stack_size - retaddr_off, RV_REG_RA, ctx);
+ /* store return address of parent function */
+ emit_sd(RV_REG_SP, stack_size - 8, RV_REG_RA, ctx);
+ /* store return address of traced function */
+ if (traced_ret)
+ emit_sd(RV_REG_SP, stack_size - 16, RV_REG_T0, ctx);
emit_sd(RV_REG_SP, stack_size - fp_off, RV_REG_FP, ctx);
emit_addi(RV_REG_FP, RV_REG_SP, stack_size, ctx);
@@ -890,7 +871,7 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im,
/* skip to actual body of traced function */
if (flags & BPF_TRAMP_F_SKIP_FRAME)
- orig_call += 16;
+ orig_call += 8;
Use the define above so it's obvious what you're skipping.
if (flags & BPF_TRAMP_F_CALL_ORIG) {
emit_imm(RV_REG_A0, (const s64)im, ctx);
@@ -962,22 +943,25 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im,
if (flags & BPF_TRAMP_F_RESTORE_REGS)
restore_args(nregs, args_off, ctx);
- if (save_ret)
+ if (save_retval)
emit_ld(RV_REG_A0, -retval_off, RV_REG_FP, ctx);
emit_ld(RV_REG_S1, -sreg_off, RV_REG_FP, ctx);
- if (flags & BPF_TRAMP_F_SKIP_FRAME)
- /* return address of parent function */
+ if (traced_ret) {
+ /* restore return address of parent function */
emit_ld(RV_REG_RA, stack_size - 8, RV_REG_SP, ctx);
- else
- /* return address of traced function */
- emit_ld(RV_REG_RA, stack_size - retaddr_off, RV_REG_SP, ctx);
+ /* restore return address of traced function */
+ emit_ld(RV_REG_T0, stack_size - 16, RV_REG_SP, ctx);
+ } else {
+ /* restore return address of parent function */
+ emit_ld(RV_REG_T0, stack_size - 8, RV_REG_SP, ctx);
+ }
emit_ld(RV_REG_FP, stack_size - fp_off, RV_REG_SP, ctx);
emit_addi(RV_REG_SP, RV_REG_SP, stack_size, ctx);
- emit_jalr(RV_REG_ZERO, RV_REG_RA, 0, ctx);
+ emit_jalr(RV_REG_ZERO, RV_REG_T0, 0, ctx);
ret = ctx->ninsns;
out:
@@ -1664,7 +1648,7 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
void bpf_jit_build_prologue(struct rv_jit_context *ctx)
{
- int i, stack_adjust = 0, store_offset, bpf_stack_adjust;
+ int stack_adjust = 0, store_offset, bpf_stack_adjust;
bpf_stack_adjust = round_up(ctx->prog->aux->stack_depth, 16);
if (bpf_stack_adjust)
@@ -1691,9 +1675,9 @@ void bpf_jit_build_prologue(struct rv_jit_context *ctx)
store_offset = stack_adjust - 8;
- /* reserve 4 nop insns */
- for (i = 0; i < 4; i++)
- emit(rv_nop(), ctx);
+ /* 2 nops reserved for auipc+jalr pair */
+ emit(rv_nop(), ctx);
+ emit(rv_nop(), ctx);
Use the define above, instead of hardcoding two nops.
Thanks,
Björn
[1] https://github.com/riscv-non-isa/riscv-elf-psabi-doc/blob/master/riscv-cc.adoc#frame-pointer-convention