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bpf: maintain bitmasks across all active frames in __mark_chain_preci…
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…sion

Teach __mark_chain_precision logic to maintain register/stack masks
across all active frames when going from child state to parent state.
Currently this should be mostly no-op, as precision backtracking usually
bails out when encountering subprog entry/exit.

It's not very apparent from the diff due to increased indentation, but
the logic remains the same, except everything is done on specific `fr`
frame index. Calls to bt_clear_reg() and bt_clear_slot() are replaced
with frame-specific bt_clear_frame_reg() and bt_clear_frame_slot(),
where frame index is passed explicitly, instead of using current frame
number.

We also adjust logging to emit affected frame number. And we also add
better logging of human-readable register and stack slot masks, similar
to previous patch.

Signed-off-by: Andrii Nakryiko <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>
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anakryiko authored and Alexei Starovoitov committed May 5, 2023
1 parent d9439c2 commit 1ef22b6
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Showing 2 changed files with 62 additions and 56 deletions.
100 changes: 53 additions & 47 deletions kernel/bpf/verifier.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -3736,7 +3736,7 @@ static int __mark_chain_precision(struct bpf_verifier_env *env, int frame, int r
struct bpf_func_state *func;
struct bpf_reg_state *reg;
bool skip_first = true;
int i, err;
int i, fr, err;

if (!env->bpf_capable)
return 0;
Expand Down Expand Up @@ -3845,56 +3845,62 @@ static int __mark_chain_precision(struct bpf_verifier_env *env, int frame, int r
if (!st)
break;

func = st->frame[frame];
bitmap_from_u64(mask, bt_reg_mask(bt));
for_each_set_bit(i, mask, 32) {
reg = &func->regs[i];
if (reg->type != SCALAR_VALUE) {
bt_clear_reg(bt, i);
continue;
for (fr = bt->frame; fr >= 0; fr--) {
func = st->frame[fr];
bitmap_from_u64(mask, bt_frame_reg_mask(bt, fr));
for_each_set_bit(i, mask, 32) {
reg = &func->regs[i];
if (reg->type != SCALAR_VALUE) {
bt_clear_frame_reg(bt, fr, i);
continue;
}
if (reg->precise)
bt_clear_frame_reg(bt, fr, i);
else
reg->precise = true;
}
if (reg->precise)
bt_clear_reg(bt, i);
else
reg->precise = true;
}

bitmap_from_u64(mask, bt_stack_mask(bt));
for_each_set_bit(i, mask, 64) {
if (i >= func->allocated_stack / BPF_REG_SIZE) {
/* the sequence of instructions:
* 2: (bf) r3 = r10
* 3: (7b) *(u64 *)(r3 -8) = r0
* 4: (79) r4 = *(u64 *)(r10 -8)
* doesn't contain jmps. It's backtracked
* as a single block.
* During backtracking insn 3 is not recognized as
* stack access, so at the end of backtracking
* stack slot fp-8 is still marked in stack_mask.
* However the parent state may not have accessed
* fp-8 and it's "unallocated" stack space.
* In such case fallback to conservative.
*/
mark_all_scalars_precise(env, st);
bt_reset(bt);
return 0;
}
bitmap_from_u64(mask, bt_frame_stack_mask(bt, fr));
for_each_set_bit(i, mask, 64) {
if (i >= func->allocated_stack / BPF_REG_SIZE) {
/* the sequence of instructions:
* 2: (bf) r3 = r10
* 3: (7b) *(u64 *)(r3 -8) = r0
* 4: (79) r4 = *(u64 *)(r10 -8)
* doesn't contain jmps. It's backtracked
* as a single block.
* During backtracking insn 3 is not recognized as
* stack access, so at the end of backtracking
* stack slot fp-8 is still marked in stack_mask.
* However the parent state may not have accessed
* fp-8 and it's "unallocated" stack space.
* In such case fallback to conservative.
*/
mark_all_scalars_precise(env, st);
bt_reset(bt);
return 0;
}

if (!is_spilled_scalar_reg(&func->stack[i])) {
bt_clear_slot(bt, i);
continue;
if (!is_spilled_scalar_reg(&func->stack[i])) {
bt_clear_frame_slot(bt, fr, i);
continue;
}
reg = &func->stack[i].spilled_ptr;
if (reg->precise)
bt_clear_frame_slot(bt, fr, i);
else
reg->precise = true;
}
if (env->log.level & BPF_LOG_LEVEL2) {
fmt_reg_mask(env->tmp_str_buf, TMP_STR_BUF_LEN,
bt_frame_reg_mask(bt, fr));
verbose(env, "mark_precise: frame%d: parent state regs=%s ",
fr, env->tmp_str_buf);
fmt_stack_mask(env->tmp_str_buf, TMP_STR_BUF_LEN,
bt_frame_stack_mask(bt, fr));
verbose(env, "stack=%s: ", env->tmp_str_buf);
print_verifier_state(env, func, true);
}
reg = &func->stack[i].spilled_ptr;
if (reg->precise)
bt_clear_slot(bt, i);
else
reg->precise = true;
}
if (env->log.level & BPF_LOG_LEVEL2) {
verbose(env, "parent %s regs=%x stack=%llx marks:",
!bt_empty(bt) ? "didn't have" : "already had",
bt_reg_mask(bt), bt_stack_mask(bt));
print_verifier_state(env, func, true);
}

if (bt_empty(bt))
Expand Down
18 changes: 9 additions & 9 deletions tools/testing/selftests/bpf/verifier/precise.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -44,7 +44,7 @@
mark_precise: frame0: regs=r2 stack= before 23\
mark_precise: frame0: regs=r2 stack= before 22\
mark_precise: frame0: regs=r2 stack= before 20\
parent didn't have regs=4 stack=0 marks:\
mark_precise: frame0: parent state regs=r2 stack=:\
mark_precise: frame0: last_idx 19 first_idx 10\
mark_precise: frame0: regs=r2 stack= before 19\
mark_precise: frame0: regs=r9 stack= before 18\
Expand All @@ -55,7 +55,7 @@
mark_precise: frame0: regs=r9 stack= before 12\
mark_precise: frame0: regs=r9 stack= before 11\
mark_precise: frame0: regs=r9 stack= before 10\
parent already had regs=0 stack=0 marks:",
mark_precise: frame0: parent state regs= stack=:",
},
{
"precise: test 2",
Expand Down Expand Up @@ -104,15 +104,15 @@
mark_precise: frame0: regs=r2 stack= before 24\
mark_precise: frame0: regs=r2 stack= before 23\
mark_precise: frame0: regs=r2 stack= before 22\
parent didn't have regs=4 stack=0 marks:\
mark_precise: frame0: parent state regs=r2 stack=:\
mark_precise: frame0: last_idx 20 first_idx 20\
mark_precise: frame0: regs=r2 stack= before 20\
parent didn't have regs=4 stack=0 marks:\
mark_precise: frame0: parent state regs=r2 stack=:\
mark_precise: frame0: last_idx 19 first_idx 17\
mark_precise: frame0: regs=r2 stack= before 19\
mark_precise: frame0: regs=r9 stack= before 18\
mark_precise: frame0: regs=r8,r9 stack= before 17\
parent already had regs=0 stack=0 marks:",
mark_precise: frame0: parent state regs= stack=:",
},
{
"precise: cross frame pruning",
Expand Down Expand Up @@ -153,14 +153,14 @@
.prog_type = BPF_PROG_TYPE_XDP,
.flags = BPF_F_TEST_STATE_FREQ,
.errstr = "mark_precise: frame0: last_idx 5 first_idx 5\
parent didn't have regs=10 stack=0 marks:\
mark_precise: frame0: parent state regs=r4 stack=:\
mark_precise: frame0: last_idx 4 first_idx 2\
mark_precise: frame0: regs=r4 stack= before 4\
mark_precise: frame0: regs=r4 stack= before 3\
mark_precise: frame0: regs= stack=-8 before 2\
mark_precise: frame0: falling back to forcing all scalars precise\
mark_precise: frame0: last_idx 5 first_idx 5\
parent didn't have regs=1 stack=0 marks:",
mark_precise: frame0: parent state regs=r0 stack=:",
.result = VERBOSE_ACCEPT,
.retval = -1,
},
Expand All @@ -179,7 +179,7 @@
.prog_type = BPF_PROG_TYPE_XDP,
.flags = BPF_F_TEST_STATE_FREQ,
.errstr = "mark_precise: frame0: last_idx 6 first_idx 6\
parent didn't have regs=10 stack=0 marks:\
mark_precise: frame0: parent state regs=r4 stack=:\
mark_precise: frame0: last_idx 5 first_idx 3\
mark_precise: frame0: regs=r4 stack= before 5\
mark_precise: frame0: regs=r4 stack= before 4\
Expand All @@ -188,7 +188,7 @@
force_precise: frame0: forcing r0 to be precise\
force_precise: frame0: forcing r0 to be precise\
mark_precise: frame0: last_idx 6 first_idx 6\
parent didn't have regs=1 stack=0 marks:\
mark_precise: frame0: parent state regs=r0 stack=:\
mark_precise: frame0: last_idx 5 first_idx 3\
mark_precise: frame0: regs=r0 stack= before 5",
.result = VERBOSE_ACCEPT,
Expand Down

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