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bpf: verifier: mark verified-insn with sub-register zext flag 

eBPF ISA specification requires high 32-bit cleared when low 32-bit
sub-register is written. This applies to destination register of ALU32 etc.
JIT back-ends must guarantee this semantic when doing code-gen. x86_64 and
AArch64 ISA has the same semantics, so the corresponding JIT back-end
doesn't need to do extra work.

However, 32-bit arches (arm, x86, nfp etc.) and some other 64-bit arches
(PowerPC, SPARC etc) need to do explicit zero extension to meet this
requirement, otherwise code like the following will fail.

  u64_value = (u64) u32_value
  ... other uses of u64_value

This is because compiler could exploit the semantic described above and
save those zero extensions for extending u32_value to u64_value, these JIT
back-ends are expected to guarantee this through inserting extra zero
extensions which however could be a significant increase on the code size.
Some benchmarks show there could be ~40% sub-register writes out of total
insns, meaning at least ~40% extra code-gen.

One observation is these extra zero extensions are not always necessary.
Take above code snippet for example, it is possible u32_value will never be
casted into a u64, the value of high 32-bit of u32_value then could be
ignored and extra zero extension could be eliminated.

This patch implements this idea, insns defining sub-registers will be
marked when the high 32-bit of the defined sub-register matters. For
those unmarked insns, it is safe to eliminate high 32-bit clearnace for
them.

Algo:
 - Split read flags into READ32 and READ64.

 - Record index of insn that does sub-register write. Keep the index inside
   reg state and update it during verifier insn walking.

 - A full register read on a sub-register marks its definition insn as
   needing zero extension on dst register.

   A new sub-register write overrides the old one.

 - When propagating read64 during path pruning, also mark any insn defining
   a sub-register that is read in the pruned path as full-register.

Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Jiong Wang 2019-05-24 23:25:12 +01:00 committed by Alexei Starovoitov
parent a08acd118d
commit 5327ed3d44
2 changed files with 171 additions and 16 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

14
include/linux/bpf_verifier.h
View File

@ -36,9 +36,11 @@
*/
enum bpf_reg_liveness {
REG_LIVE_NONE = 0, /* reg hasn't been read or written this branch */
REG_LIVE_READ, /* reg was read, so we're sensitive to initial value */
REG_LIVE_WRITTEN, /* reg was written first, screening off later reads */
REG_LIVE_DONE = 4, /* liveness won't be updating this register anymore */
REG_LIVE_READ32 = 0x1, /* reg was read, so we're sensitive to initial value */
REG_LIVE_READ64 = 0x2, /* likewise, but full 64-bit content matters */
REG_LIVE_READ = REG_LIVE_READ32 | REG_LIVE_READ64,
REG_LIVE_WRITTEN = 0x4, /* reg was written first, screening off later reads */
REG_LIVE_DONE = 0x8, /* liveness won't be updating this register anymore */
};
struct bpf_reg_state {
@ -131,6 +133,11 @@ struct bpf_reg_state {
* pointing to bpf_func_state.
*/
u32 frameno;
/* Tracks subreg definition. The stored value is the insn_idx of the
* writing insn. This is safe because subreg_def is used before any insn
* patching which only happens after main verification finished.
*/
s32 subreg_def;
enum bpf_reg_liveness live;
};
@ -233,6 +240,7 @@ struct bpf_insn_aux_data {
int ctx_field_size; /* the ctx field size for load insn, maybe 0 */
int sanitize_stack_off; /* stack slot to be cleared */
bool seen; /* this insn was processed by the verifier */
bool zext_dst; /* this insn zero extends dst reg */
u8 alu_state; /* used in combination with alu_limit */
bool prune_point;
unsigned int orig_idx; /* original instruction index */

173
kernel/bpf/verifier.c
View File

@ -982,6 +982,7 @@ static void mark_reg_not_init(struct bpf_verifier_env *env,
__mark_reg_not_init(regs + regno);
}
#define DEF_NOT_SUBREG (0)
static void init_reg_state(struct bpf_verifier_env *env,
struct bpf_func_state *state)
{
@ -992,6 +993,7 @@ static void init_reg_state(struct bpf_verifier_env *env,
mark_reg_not_init(env, regs, i);
regs[i].live = REG_LIVE_NONE;
regs[i].parent = NULL;
regs[i].subreg_def = DEF_NOT_SUBREG;
}
/* frame pointer */
@ -1137,7 +1139,7 @@ next:
*/
static int mark_reg_read(struct bpf_verifier_env *env,
const struct bpf_reg_state *state,
struct bpf_reg_state *parent)
struct bpf_reg_state *parent, u8 flag)
{
bool writes = parent == state->parent; /* Observe write marks */
int cnt = 0;
@ -1152,17 +1154,26 @@ static int mark_reg_read(struct bpf_verifier_env *env,
parent->var_off.value, parent->off);
return -EFAULT;
}
if (parent->live & REG_LIVE_READ)
/* The first condition is more likely to be true than the
* second, checked it first.
*/
if ((parent->live & REG_LIVE_READ) == flag ||
parent->live & REG_LIVE_READ64)
/* The parentage chain never changes and
* this parent was already marked as LIVE_READ.
* There is no need to keep walking the chain again and
* keep re-marking all parents as LIVE_READ.
* This case happens when the same register is read
* multiple times without writes into it in-between.
* Also, if parent has the stronger REG_LIVE_READ64 set,
* then no need to set the weak REG_LIVE_READ32.
*/
break;
/* ... then we depend on parent's value */
parent->live |= REG_LIVE_READ;
parent->live |= flag;
/* REG_LIVE_READ64 overrides REG_LIVE_READ32. */
if (flag == REG_LIVE_READ64)
parent->live &= ~REG_LIVE_READ32;
state = parent;
parent = state->parent;
writes = true;
@ -1174,12 +1185,111 @@ static int mark_reg_read(struct bpf_verifier_env *env,
return 0;
}
/* This function is supposed to be used by the following 32-bit optimization
* code only. It returns TRUE if the source or destination register operates
* on 64-bit, otherwise return FALSE.
*/
static bool is_reg64(struct bpf_verifier_env *env, struct bpf_insn *insn,
u32 regno, struct bpf_reg_state *reg, enum reg_arg_type t)
{
u8 code, class, op;
code = insn->code;
class = BPF_CLASS(code);
op = BPF_OP(code);
if (class == BPF_JMP) {
/* BPF_EXIT for "main" will reach here. Return TRUE
* conservatively.
*/
if (op == BPF_EXIT)
return true;
if (op == BPF_CALL) {
/* BPF to BPF call will reach here because of marking
* caller saved clobber with DST_OP_NO_MARK for which we
* don't care the register def because they are anyway
* marked as NOT_INIT already.
*/
if (insn->src_reg == BPF_PSEUDO_CALL)
return false;
/* Helper call will reach here because of arg type
* check, conservatively return TRUE.
*/
if (t == SRC_OP)
return true;
return false;
}
}
if (class == BPF_ALU64 || class == BPF_JMP ||
/* BPF_END always use BPF_ALU class. */
(class == BPF_ALU && op == BPF_END && insn->imm == 64))
return true;
if (class == BPF_ALU || class == BPF_JMP32)
return false;
if (class == BPF_LDX) {
if (t != SRC_OP)
return BPF_SIZE(code) == BPF_DW;
/* LDX source must be ptr. */
return true;
}
if (class == BPF_STX) {
if (reg->type != SCALAR_VALUE)
return true;
return BPF_SIZE(code) == BPF_DW;
}
if (class == BPF_LD) {
u8 mode = BPF_MODE(code);
/* LD_IMM64 */
if (mode == BPF_IMM)
return true;
/* Both LD_IND and LD_ABS return 32-bit data. */
if (t != SRC_OP)
return false;
/* Implicit ctx ptr. */
if (regno == BPF_REG_6)
return true;
/* Explicit source could be any width. */
return true;
}
if (class == BPF_ST)
/* The only source register for BPF_ST is a ptr. */
return true;
/* Conservatively return true at default. */
return true;
}
static void mark_insn_zext(struct bpf_verifier_env *env,
struct bpf_reg_state *reg)
{
s32 def_idx = reg->subreg_def;
if (def_idx == DEF_NOT_SUBREG)
return;
env->insn_aux_data[def_idx - 1].zext_dst = true;
/* The dst will be zero extended, so won't be sub-register anymore. */
reg->subreg_def = DEF_NOT_SUBREG;
}
static int check_reg_arg(struct bpf_verifier_env *env, u32 regno,
enum reg_arg_type t)
{
struct bpf_verifier_state *vstate = env->cur_state;
struct bpf_func_state *state = vstate->frame[vstate->curframe];
struct bpf_insn *insn = env->prog->insnsi + env->insn_idx;
struct bpf_reg_state *reg, *regs = state->regs;
bool rw64;
if (regno >= MAX_BPF_REG) {
verbose(env, "R%d is invalid\n", regno);
@ -1187,6 +1297,7 @@ static int check_reg_arg(struct bpf_verifier_env *env, u32 regno,
}
reg = &regs[regno];
rw64 = is_reg64(env, insn, regno, reg, t);
if (t == SRC_OP) {
/* check whether register used as source operand can be read */
if (reg->type == NOT_INIT) {
@ -1197,7 +1308,11 @@ static int check_reg_arg(struct bpf_verifier_env *env, u32 regno,
if (regno == BPF_REG_FP)
return 0;
return mark_reg_read(env, reg, reg->parent);
if (rw64)
mark_insn_zext(env, reg);
return mark_reg_read(env, reg, reg->parent,
rw64 ? REG_LIVE_READ64 : REG_LIVE_READ32);
} else {
/* check whether register used as dest operand can be written to */
if (regno == BPF_REG_FP) {
@ -1205,6 +1320,7 @@ static int check_reg_arg(struct bpf_verifier_env *env, u32 regno,
return -EACCES;
}
reg->live |= REG_LIVE_WRITTEN;
reg->subreg_def = rw64 ? DEF_NOT_SUBREG : env->insn_idx + 1;
if (t == DST_OP)
mark_reg_unknown(env, regs, regno);
}
@ -1384,7 +1500,8 @@ static int check_stack_read(struct bpf_verifier_env *env,
state->regs[value_regno].live |= REG_LIVE_WRITTEN;
}
mark_reg_read(env, &reg_state->stack[spi].spilled_ptr,
reg_state->stack[spi].spilled_ptr.parent);
reg_state->stack[spi].spilled_ptr.parent,
REG_LIVE_READ64);
return 0;
} else {
int zeros = 0;
@ -1401,7 +1518,8 @@ static int check_stack_read(struct bpf_verifier_env *env,
return -EACCES;
}
mark_reg_read(env, &reg_state->stack[spi].spilled_ptr,
reg_state->stack[spi].spilled_ptr.parent);
reg_state->stack[spi].spilled_ptr.parent,
REG_LIVE_READ64);
if (value_regno >= 0) {
if (zeros == size) {
/* any size read into register is zero extended,
@ -2110,6 +2228,12 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn
value_regno);
if (reg_type_may_be_null(reg_type))
regs[value_regno].id = ++env->id_gen;
/* A load of ctx field could have different
* actual load size with the one encoded in the
* insn. When the dst is PTR, it is for sure not
* a sub-register.
*/
regs[value_regno].subreg_def = DEF_NOT_SUBREG;
}
regs[value_regno].type = reg_type;
}
@ -2369,7 +2493,8 @@ mark:
* the whole slot to be marked as 'read'
*/
mark_reg_read(env, &state->stack[spi].spilled_ptr,
state->stack[spi].spilled_ptr.parent);
state->stack[spi].spilled_ptr.parent,
REG_LIVE_READ64);
}
return update_stack_depth(env, state, min_off);
}
@ -3333,6 +3458,9 @@ static int check_helper_call(struct bpf_verifier_env *env, int func_id, int insn
check_reg_arg(env, caller_saved[i], DST_OP_NO_MARK);
}
/* helper call returns 64-bit value. */
regs[BPF_REG_0].subreg_def = DEF_NOT_SUBREG;
/* update return register (already marked as written above) */
if (fn->ret_type == RET_INTEGER) {
/* sets type to SCALAR_VALUE */
@ -4264,6 +4392,7 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
*/
*dst_reg = *src_reg;
dst_reg->live |= REG_LIVE_WRITTEN;
dst_reg->subreg_def = DEF_NOT_SUBREG;
} else {
/* R1 = (u32) R2 */
if (is_pointer_value(env, insn->src_reg)) {
@ -4274,6 +4403,7 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
} else if (src_reg->type == SCALAR_VALUE) {
*dst_reg = *src_reg;
dst_reg->live |= REG_LIVE_WRITTEN;
dst_reg->subreg_def = env->insn_idx + 1;
} else {
mark_reg_unknown(env, regs,
insn->dst_reg);
@ -5353,6 +5483,8 @@ static int check_ld_abs(struct bpf_verifier_env *env, struct bpf_insn *insn)
* Already marked as written above.
*/
mark_reg_unknown(env, regs, BPF_REG_0);
/* ld_abs load up to 32-bit skb data. */
regs[BPF_REG_0].subreg_def = env->insn_idx + 1;
return 0;
}
@ -6309,20 +6441,33 @@ static bool states_equal(struct bpf_verifier_env *env,
return true;
}
/* Return 0 if no propagation happened. Return negative error code if error
* happened. Otherwise, return the propagated bit.
*/
static int propagate_liveness_reg(struct bpf_verifier_env *env,
struct bpf_reg_state *reg,
struct bpf_reg_state *parent_reg)
{
u8 parent_flag = parent_reg->live & REG_LIVE_READ;
u8 flag = reg->live & REG_LIVE_READ;
int err;
if (parent_reg->live & REG_LIVE_READ || !(reg->live & REG_LIVE_READ))
/* When comes here, read flags of PARENT_REG or REG could be any of
* REG_LIVE_READ64, REG_LIVE_READ32, REG_LIVE_NONE. There is no need
* of propagation if PARENT_REG has strongest REG_LIVE_READ64.
*/
if (parent_flag == REG_LIVE_READ64 ||
/* Or if there is no read flag from REG. */
!flag ||
/* Or if the read flag from REG is the same as PARENT_REG. */
parent_flag == flag)
return 0;
err = mark_reg_read(env, reg, parent_reg);
err = mark_reg_read(env, reg, parent_reg, flag);
if (err)
return err;
return 0;
return flag;
}
/* A write screens off any subsequent reads; but write marks come from the
@ -6356,8 +6501,10 @@ static int propagate_liveness(struct bpf_verifier_env *env,
for (i = frame < vstate->curframe ? BPF_REG_6 : 0; i < BPF_REG_FP; i++) {
err = propagate_liveness_reg(env, &state_reg[i],
&parent_reg[i]);
if (err)
if (err < 0)
return err;
if (err == REG_LIVE_READ64)
mark_insn_zext(env, &parent_reg[i]);
}
/* Propagate stack slots. */
@ -6367,11 +6514,11 @@ static int propagate_liveness(struct bpf_verifier_env *env,
state_reg = &state->stack[i].spilled_ptr;
err = propagate_liveness_reg(env, state_reg,
parent_reg);
if (err)
if (err < 0)
return err;
}
}
return err;
return 0;
}
static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)