* [PATCH bpf v2] libbpf : make libbpf_num_possible_cpus function thread safe
@ 2019-07-31 22:10 Takshak Chahande
2019-07-31 22:17 ` [Potential Spoof] " Andrey Ignatov
` (2 more replies)
0 siblings, 3 replies; 4+ messages in thread
From: Takshak Chahande @ 2019-07-31 22:10 UTC (permalink / raw)
To: netdev; +Cc: ast, daniel, rdna, ctakshak, kernel-team, hechaol, jakub.kicinski
Having static variable `cpus` in libbpf_num_possible_cpus function
without guarding it with mutex makes this function thread-unsafe.
If multiple threads accessing this function, in the current form; it
leads to incrementing the static variable value `cpus` in the multiple
of total available CPUs.
Used local stack variable to calculate the number of possible CPUs and
then updated the static variable using WRITE_ONCE().
Changes since v1:
* added stack variable to calculate cpus
* serialized static variable update using WRITE_ONCE()
* fixed Fixes tag
Fixes: 6446b3155521 ("bpf: add a new API libbpf_num_possible_cpus()")
Signed-off-by: Takshak Chahande <ctakshak@fb.com>
---
tools/lib/bpf/libbpf.c | 18 +++++++++++-------
1 file changed, 11 insertions(+), 7 deletions(-)
diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 6718d0b90130..2e84fa5b8479 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -4995,13 +4995,15 @@ int libbpf_num_possible_cpus(void)
static const char *fcpu = "/sys/devices/system/cpu/possible";
int len = 0, n = 0, il = 0, ir = 0;
unsigned int start = 0, end = 0;
+ int tmp_cpus = 0;
static int cpus;
char buf[128];
int error = 0;
int fd = -1;
- if (cpus > 0)
- return cpus;
+ tmp_cpus = READ_ONCE(cpus);
+ if (tmp_cpus > 0)
+ return tmp_cpus;
fd = open(fcpu, O_RDONLY);
if (fd < 0) {
@@ -5024,7 +5026,7 @@ int libbpf_num_possible_cpus(void)
}
buf[len] = '\0';
- for (ir = 0, cpus = 0; ir <= len; ir++) {
+ for (ir = 0, tmp_cpus = 0; ir <= len; ir++) {
/* Each sub string separated by ',' has format \d+-\d+ or \d+ */
if (buf[ir] == ',' || buf[ir] == '\0') {
buf[ir] = '\0';
@@ -5036,13 +5038,15 @@ int libbpf_num_possible_cpus(void)
} else if (n == 1) {
end = start;
}
- cpus += end - start + 1;
+ tmp_cpus += end - start + 1;
il = ir + 1;
}
}
- if (cpus <= 0) {
- pr_warning("Invalid #CPUs %d from %s\n", cpus, fcpu);
+ if (tmp_cpus <= 0) {
+ pr_warning("Invalid #CPUs %d from %s\n", tmp_cpus, fcpu);
return -EINVAL;
}
- return cpus;
+
+ WRITE_ONCE(cpus, tmp_cpus);
+ return tmp_cpus;
}
--
2.17.1
^ permalink raw reply related [flat|nested] 4+ messages in thread
* Re: [Potential Spoof] [PATCH bpf v2] libbpf : make libbpf_num_possible_cpus function thread safe
2019-07-31 22:10 [PATCH bpf v2] libbpf : make libbpf_num_possible_cpus function thread safe Takshak Chahande
@ 2019-07-31 22:17 ` Andrey Ignatov
2019-07-31 22:31 ` Jakub Kicinski
2019-08-01 3:40 ` Alexei Starovoitov
2 siblings, 0 replies; 4+ messages in thread
From: Andrey Ignatov @ 2019-07-31 22:17 UTC (permalink / raw)
To: Takshak Chahande
Cc: netdev, ast, daniel, Kernel Team, Hechao Li, jakub.kicinski
Takshak Chahande <ctakshak@fb.com> [Wed, 2019-07-31 15:11 -0700]:
> Having static variable `cpus` in libbpf_num_possible_cpus function
> without guarding it with mutex makes this function thread-unsafe.
>
> If multiple threads accessing this function, in the current form; it
> leads to incrementing the static variable value `cpus` in the multiple
> of total available CPUs.
>
> Used local stack variable to calculate the number of possible CPUs and
> then updated the static variable using WRITE_ONCE().
>
> Changes since v1:
> * added stack variable to calculate cpus
> * serialized static variable update using WRITE_ONCE()
> * fixed Fixes tag
This "Changes" section should be after "---" line not to be included in
the final commit message.
Not sure if resubmit is needed because of it, but other than this looks
good to me.
Acked-by: Andrey Ignatov <rdna@fb.com>
> Fixes: 6446b3155521 ("bpf: add a new API libbpf_num_possible_cpus()")
> Signed-off-by: Takshak Chahande <ctakshak@fb.com>
> ---
> tools/lib/bpf/libbpf.c | 18 +++++++++++-------
> 1 file changed, 11 insertions(+), 7 deletions(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 6718d0b90130..2e84fa5b8479 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -4995,13 +4995,15 @@ int libbpf_num_possible_cpus(void)
> static const char *fcpu = "/sys/devices/system/cpu/possible";
> int len = 0, n = 0, il = 0, ir = 0;
> unsigned int start = 0, end = 0;
> + int tmp_cpus = 0;
> static int cpus;
> char buf[128];
> int error = 0;
> int fd = -1;
>
> - if (cpus > 0)
> - return cpus;
> + tmp_cpus = READ_ONCE(cpus);
> + if (tmp_cpus > 0)
> + return tmp_cpus;
>
> fd = open(fcpu, O_RDONLY);
> if (fd < 0) {
> @@ -5024,7 +5026,7 @@ int libbpf_num_possible_cpus(void)
> }
> buf[len] = '\0';
>
> - for (ir = 0, cpus = 0; ir <= len; ir++) {
> + for (ir = 0, tmp_cpus = 0; ir <= len; ir++) {
> /* Each sub string separated by ',' has format \d+-\d+ or \d+ */
> if (buf[ir] == ',' || buf[ir] == '\0') {
> buf[ir] = '\0';
> @@ -5036,13 +5038,15 @@ int libbpf_num_possible_cpus(void)
> } else if (n == 1) {
> end = start;
> }
> - cpus += end - start + 1;
> + tmp_cpus += end - start + 1;
> il = ir + 1;
> }
> }
> - if (cpus <= 0) {
> - pr_warning("Invalid #CPUs %d from %s\n", cpus, fcpu);
> + if (tmp_cpus <= 0) {
> + pr_warning("Invalid #CPUs %d from %s\n", tmp_cpus, fcpu);
> return -EINVAL;
> }
> - return cpus;
> +
> + WRITE_ONCE(cpus, tmp_cpus);
> + return tmp_cpus;
> }
> --
> 2.17.1
>
--
Andrey Ignatov
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [PATCH bpf v2] libbpf : make libbpf_num_possible_cpus function thread safe
2019-07-31 22:10 [PATCH bpf v2] libbpf : make libbpf_num_possible_cpus function thread safe Takshak Chahande
2019-07-31 22:17 ` [Potential Spoof] " Andrey Ignatov
@ 2019-07-31 22:31 ` Jakub Kicinski
2019-08-01 3:40 ` Alexei Starovoitov
2 siblings, 0 replies; 4+ messages in thread
From: Jakub Kicinski @ 2019-07-31 22:31 UTC (permalink / raw)
To: Takshak Chahande; +Cc: netdev, ast, daniel, rdna, kernel-team, hechaol
On Wed, 31 Jul 2019 15:10:55 -0700, Takshak Chahande wrote:
> Having static variable `cpus` in libbpf_num_possible_cpus function
> without guarding it with mutex makes this function thread-unsafe.
>
> If multiple threads accessing this function, in the current form; it
> leads to incrementing the static variable value `cpus` in the multiple
> of total available CPUs.
>
> Used local stack variable to calculate the number of possible CPUs and
> then updated the static variable using WRITE_ONCE().
>
> Changes since v1:
> * added stack variable to calculate cpus
> * serialized static variable update using WRITE_ONCE()
> * fixed Fixes tag
>
> Fixes: 6446b3155521 ("bpf: add a new API libbpf_num_possible_cpus()")
> Signed-off-by: Takshak Chahande <ctakshak@fb.com>
Perhaps we would have a little less code churn if the static variable
was renamed (e.g. to saved_cpus), but functionally looks good, so:
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
(FWIW I think Andrey's comment does not apply to the networking and BPF
trees so if you respin please keep the changelog in the commit message.)
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [PATCH bpf v2] libbpf : make libbpf_num_possible_cpus function thread safe
2019-07-31 22:10 [PATCH bpf v2] libbpf : make libbpf_num_possible_cpus function thread safe Takshak Chahande
2019-07-31 22:17 ` [Potential Spoof] " Andrey Ignatov
2019-07-31 22:31 ` Jakub Kicinski
@ 2019-08-01 3:40 ` Alexei Starovoitov
2 siblings, 0 replies; 4+ messages in thread
From: Alexei Starovoitov @ 2019-08-01 3:40 UTC (permalink / raw)
To: Takshak Chahande
Cc: Network Development, Alexei Starovoitov, Daniel Borkmann,
Andrey Ignatov, Kernel Team, hechaol, Jakub Kicinski
On Wed, Jul 31, 2019 at 3:11 PM Takshak Chahande <ctakshak@fb.com> wrote:
>
> Having static variable `cpus` in libbpf_num_possible_cpus function
> without guarding it with mutex makes this function thread-unsafe.
>
> If multiple threads accessing this function, in the current form; it
> leads to incrementing the static variable value `cpus` in the multiple
> of total available CPUs.
>
> Used local stack variable to calculate the number of possible CPUs and
> then updated the static variable using WRITE_ONCE().
>
> Changes since v1:
> * added stack variable to calculate cpus
> * serialized static variable update using WRITE_ONCE()
> * fixed Fixes tag
>
> Fixes: 6446b3155521 ("bpf: add a new API libbpf_num_possible_cpus()")
> Signed-off-by: Takshak Chahande <ctakshak@fb.com>
Applied. Thanks.
Thanks for keeping 'changes since v1 part' as part of git history.
^ permalink raw reply [flat|nested] 4+ messages in thread
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2019-07-31 22:10 [PATCH bpf v2] libbpf : make libbpf_num_possible_cpus function thread safe Takshak Chahande
2019-07-31 22:17 ` [Potential Spoof] " Andrey Ignatov
2019-07-31 22:31 ` Jakub Kicinski
2019-08-01 3:40 ` Alexei Starovoitov
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