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bpf: zero-fill re-used per-cpu map element
Zero-fill element values for all other cpus than current, just as when not using prealloc. This is the only way the bpf program can ensure known initial values for all cpus ('onallcpus' cannot be set when coming from the bpf program). The scenario is: bpf program inserts some elements in a per-cpu map, then deletes some (or userspace does). When later adding new elements using bpf_map_update_elem(), the bpf program can only set the value of the new elements for the current cpu. When prealloc is enabled, previously deleted elements are re-used. Without the fix, values for other cpus remain whatever they were when the re-used entry was previously freed. A selftest is added to validate correct operation in above scenario as well as in case of LRU per-cpu map element re-use. Fixes: 6c90598 ("bpf: pre-allocate hash map elements") Acked-by: Matthieu Baerts <[email protected]> Signed-off-by: David Verbeiren <[email protected]> Acked-by: Andrii Nakryiko <[email protected]>
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3 files changed

+275
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‎kernel/bpf/hashtab.c

Lines changed: 28 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -821,6 +821,32 @@ static void pcpu_copy_value(struct bpf_htab *htab, void __percpu *pptr,
821821
}
822822
}
823823

824+
static void pcpu_init_value(struct bpf_htab *htab, void __percpu *pptr,
825+
void *value, bool onallcpus)
826+
{
827+
/* When using prealloc and not setting the initial value on all cpus,
828+
* zero-fill element values for other cpus (just as what happens when
829+
* not using prealloc). Otherwise, bpf program has no way to ensure
830+
* known initial values for cpus other than current one
831+
* (onallcpus=false always when coming from bpf prog).
832+
*/
833+
if (htab_is_prealloc(htab) && !onallcpus) {
834+
u32 size = round_up(htab->map.value_size, 8);
835+
int current_cpu = raw_smp_processor_id();
836+
int cpu;
837+
838+
for_each_possible_cpu(cpu) {
839+
if (cpu == current_cpu)
840+
bpf_long_memcpy(per_cpu_ptr(pptr, cpu), value,
841+
size);
842+
else
843+
memset(per_cpu_ptr(pptr, cpu), 0, size);
844+
}
845+
} else {
846+
pcpu_copy_value(htab, pptr, value, onallcpus);
847+
}
848+
}
849+
824850
static bool fd_htab_map_needs_adjust(const struct bpf_htab *htab)
825851
{
826852
return htab->map.map_type == BPF_MAP_TYPE_HASH_OF_MAPS &&
@@ -891,7 +917,7 @@ static struct htab_elem *alloc_htab_elem(struct bpf_htab *htab, void *key,
891917
}
892918
}
893919

894-
pcpu_copy_value(htab, pptr, value, onallcpus);
920+
pcpu_init_value(htab, pptr, value, onallcpus);
895921

896922
if (!prealloc)
897923
htab_elem_set_ptr(l_new, key_size, pptr);
@@ -1183,7 +1209,7 @@ static int __htab_lru_percpu_map_update_elem(struct bpf_map *map, void *key,
11831209
pcpu_copy_value(htab, htab_elem_get_ptr(l_old, key_size),
11841210
value, onallcpus);
11851211
} else {
1186-
pcpu_copy_value(htab, htab_elem_get_ptr(l_new, key_size),
1212+
pcpu_init_value(htab, htab_elem_get_ptr(l_new, key_size),
11871213
value, onallcpus);
11881214
hlist_nulls_add_head_rcu(&l_new->hash_node, head);
11891215
l_new = NULL;

‎tools/testing/selftests/bpf/prog_tests/map_init.c

Lines changed: 214 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,214 @@
1+
// SPDX-License-Identifier: GPL-2.0-only
2+
/* Copyright (c) 2020 Tessares SA <http://www.tessares.net> */
3+
4+
#include <test_progs.h>
5+
#include "test_map_init.skel.h"
6+
7+
#define TEST_VALUE 0x1234
8+
#define FILL_VALUE 0xdeadbeef
9+
10+
static int nr_cpus;
11+
static int duration;
12+
13+
typedef unsigned long long map_key_t;
14+
typedef unsigned long long map_value_t;
15+
typedef struct {
16+
map_value_t v; /* padding */
17+
} __bpf_percpu_val_align pcpu_map_value_t;
18+
19+
20+
static int map_populate(int map_fd, int num)
21+
{
22+
pcpu_map_value_t value[nr_cpus];
23+
int i, err;
24+
map_key_t key;
25+
26+
for (i = 0; i < nr_cpus; i++)
27+
bpf_percpu(value, i) = FILL_VALUE;
28+
29+
for (key = 1; key <= num; key++) {
30+
err = bpf_map_update_elem(map_fd, &key, value, BPF_NOEXIST);
31+
if (!ASSERT_OK(err, "bpf_map_update_elem"))
32+
return -1;
33+
}
34+
35+
return 0;
36+
}
37+
38+
static struct test_map_init *setup(enum bpf_map_type map_type, int map_sz,
39+
int *map_fd, int populate)
40+
{
41+
struct test_map_init *skel;
42+
int err;
43+
44+
skel = test_map_init__open();
45+
if (!ASSERT_OK_PTR(skel, "skel_open"))
46+
return NULL;
47+
48+
err = bpf_map__set_type(skel->maps.hashmap1, map_type);
49+
if (!ASSERT_OK(err, "bpf_map__set_type"))
50+
goto error;
51+
52+
err = bpf_map__set_max_entries(skel->maps.hashmap1, map_sz);
53+
if (!ASSERT_OK(err, "bpf_map__set_max_entries"))
54+
goto error;
55+
56+
err = test_map_init__load(skel);
57+
if (!ASSERT_OK(err, "skel_load"))
58+
goto error;
59+
60+
*map_fd = bpf_map__fd(skel->maps.hashmap1);
61+
if (CHECK(*map_fd < 0, "bpf_map__fd", "failed\n"))
62+
goto error;
63+
64+
err = map_populate(*map_fd, populate);
65+
if (!ASSERT_OK(err, "map_populate"))
66+
goto error_map;
67+
68+
return skel;
69+
70+
error_map:
71+
close(*map_fd);
72+
error:
73+
test_map_init__destroy(skel);
74+
return NULL;
75+
}
76+
77+
/* executes bpf program that updates map with key, value */
78+
static int prog_run_insert_elem(struct test_map_init *skel, map_key_t key,
79+
map_value_t value)
80+
{
81+
struct test_map_init__bss *bss;
82+
83+
bss = skel->bss;
84+
85+
bss->inKey = key;
86+
bss->inValue = value;
87+
bss->inPid = getpid();
88+
89+
if (!ASSERT_OK(test_map_init__attach(skel), "skel_attach"))
90+
return -1;
91+
92+
/* Let tracepoint trigger */
93+
syscall(__NR_getpgid);
94+
95+
test_map_init__detach(skel);
96+
97+
return 0;
98+
}
99+
100+
static int check_values_one_cpu(pcpu_map_value_t *value, map_value_t expected)
101+
{
102+
int i, nzCnt = 0;
103+
map_value_t val;
104+
105+
for (i = 0; i < nr_cpus; i++) {
106+
val = bpf_percpu(value, i);
107+
if (val) {
108+
if (CHECK(val != expected, "map value",
109+
"unexpected for cpu %d: 0x%llx\n", i, val))
110+
return -1;
111+
nzCnt++;
112+
}
113+
}
114+
115+
if (CHECK(nzCnt != 1, "map value", "set for %d CPUs instead of 1!\n",
116+
nzCnt))
117+
return -1;
118+
119+
return 0;
120+
}
121+
122+
/* Add key=1 elem with values set for all CPUs
123+
* Delete elem key=1
124+
* Run bpf prog that inserts new key=1 elem with value=0x1234
125+
* (bpf prog can only set value for current CPU)
126+
* Lookup Key=1 and check value is as expected for all CPUs:
127+
* value set by bpf prog for one CPU, 0 for all others
128+
*/
129+
static void test_pcpu_map_init(void)
130+
{
131+
pcpu_map_value_t value[nr_cpus];
132+
struct test_map_init *skel;
133+
int map_fd, err;
134+
map_key_t key;
135+
136+
/* max 1 elem in map so insertion is forced to reuse freed entry */
137+
skel = setup(BPF_MAP_TYPE_PERCPU_HASH, 1, &map_fd, 1);
138+
if (!ASSERT_OK_PTR(skel, "prog_setup"))
139+
return;
140+
141+
/* delete element so the entry can be re-used*/
142+
key = 1;
143+
err = bpf_map_delete_elem(map_fd, &key);
144+
if (!ASSERT_OK(err, "bpf_map_delete_elem"))
145+
goto cleanup;
146+
147+
/* run bpf prog that inserts new elem, re-using the slot just freed */
148+
err = prog_run_insert_elem(skel, key, TEST_VALUE);
149+
if (!ASSERT_OK(err, "prog_run_insert_elem"))
150+
goto cleanup;
151+
152+
/* check that key=1 was re-created by bpf prog */
153+
err = bpf_map_lookup_elem(map_fd, &key, value);
154+
if (!ASSERT_OK(err, "bpf_map_lookup_elem"))
155+
goto cleanup;
156+
157+
/* and has expected values */
158+
check_values_one_cpu(value, TEST_VALUE);
159+
160+
cleanup:
161+
test_map_init__destroy(skel);
162+
}
163+
164+
/* Add key=1 and key=2 elems with values set for all CPUs
165+
* Run bpf prog that inserts new key=3 elem
166+
* (only for current cpu; other cpus should have initial value = 0)
167+
* Lookup Key=1 and check value is as expected for all CPUs
168+
*/
169+
static void test_pcpu_lru_map_init(void)
170+
{
171+
pcpu_map_value_t value[nr_cpus];
172+
struct test_map_init *skel;
173+
int map_fd, err;
174+
map_key_t key;
175+
176+
/* Set up LRU map with 2 elements, values filled for all CPUs.
177+
* With these 2 elements, the LRU map is full
178+
*/
179+
skel = setup(BPF_MAP_TYPE_LRU_PERCPU_HASH, 2, &map_fd, 2);
180+
if (!ASSERT_OK_PTR(skel, "prog_setup"))
181+
return;
182+
183+
/* run bpf prog that inserts new key=3 element, re-using LRU slot */
184+
key = 3;
185+
err = prog_run_insert_elem(skel, key, TEST_VALUE);
186+
if (!ASSERT_OK(err, "prog_run_insert_elem"))
187+
goto cleanup;
188+
189+
/* check that key=3 replaced one of earlier elements */
190+
err = bpf_map_lookup_elem(map_fd, &key, value);
191+
if (!ASSERT_OK(err, "bpf_map_lookup_elem"))
192+
goto cleanup;
193+
194+
/* and has expected values */
195+
check_values_one_cpu(value, TEST_VALUE);
196+
197+
cleanup:
198+
test_map_init__destroy(skel);
199+
}
200+
201+
void test_map_init(void)
202+
{
203+
nr_cpus = bpf_num_possible_cpus();
204+
if (nr_cpus <= 1) {
205+
printf("%s:SKIP: >1 cpu needed for this test\n", __func__);
206+
test__skip();
207+
return;
208+
}
209+
210+
if (test__start_subtest("pcpu_map_init"))
211+
test_pcpu_map_init();
212+
if (test__start_subtest("pcpu_lru_map_init"))
213+
test_pcpu_lru_map_init();
214+
}

‎tools/testing/selftests/bpf/progs/test_map_init.c

Lines changed: 33 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,33 @@
1+
// SPDX-License-Identifier: GPL-2.0
2+
/* Copyright (c) 2020 Tessares SA <http://www.tessares.net> */
3+
4+
#include "vmlinux.h"
5+
#include <bpf/bpf_helpers.h>
6+
7+
__u64 inKey = 0;
8+
__u64 inValue = 0;
9+
__u32 inPid = 0;
10+
11+
struct {
12+
__uint(type, BPF_MAP_TYPE_PERCPU_HASH);
13+
__uint(max_entries, 2);
14+
__type(key, __u64);
15+
__type(value, __u64);
16+
} hashmap1 SEC(".maps");
17+
18+
19+
SEC("tp/syscalls/sys_enter_getpgid")
20+
int sysenter_getpgid(const void *ctx)
21+
{
22+
/* Just do it for once, when called from our own test prog. This
23+
* ensures the map value is only updated for a single CPU.
24+
*/
25+
int cur_pid = bpf_get_current_pid_tgid() >> 32;
26+
27+
if (cur_pid == inPid)
28+
bpf_map_update_elem(&hashmap1, &inKey, &inValue, BPF_NOEXIST);
29+
30+
return 0;
31+
}
32+
33+
char _license[] SEC("license") = "GPL";

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