On 9/22/21 9:07 AM, Muhammad Falak R Wani wrote:
Simplify libbpf_num_possible_cpus by using sysconf(_SC_NPROCESSORS_CONF)
instead of parsing a file.
This patch is a part ([0]) of libbpf-1.0 milestone.
[0] Closes: https://github.com/libbpf/libbpf/issues/383
Signed-off-by: Muhammad Falak R Wani <falakreyaz@xxxxxxxxx>
---
tools/lib/bpf/libbpf.c | 17 ++++-------------
1 file changed, 4 insertions(+), 13 deletions(-)
diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index ef5db34bf913..f1c0abe5b58d 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -10898,25 +10898,16 @@ int parse_cpu_mask_file(const char *fcpu, bool **mask, int *mask_sz)
int libbpf_num_possible_cpus(void)
{
- static const char *fcpu = "/sys/devices/system/cpu/possible";
static int cpus;
- int err, n, i, tmp_cpus;
- bool *mask;
+ int tmp_cpus;
tmp_cpus = READ_ONCE(cpus);
if (tmp_cpus > 0)
return tmp_cpus;
- err = parse_cpu_mask_file(fcpu, &mask, &n);
- if (err)
- return libbpf_err(err);
-
- tmp_cpus = 0;
- for (i = 0; i < n; i++) {
- if (mask[i])
- tmp_cpus++;
- }
- free(mask);
+ tmp_cpus = sysconf(_SC_NPROCESSORS_CONF);
+ if (tmp_cpus < 1)
+ return libbpf_err(-EINVAL);
This approach is unfortunately broken, see also commit e00c7b216f34 ("bpf: fix
multiple issues in selftest suite and samples") for more details:
3) Current selftest suite code relies on sysconf(_SC_NPROCESSORS_CONF) for
retrieving the number of possible CPUs. This is broken at least in our
scenario and really just doesn't work.
glibc tries a number of things for retrieving _SC_NPROCESSORS_CONF.
First it tries equivalent of /sys/devices/system/cpu/cpu[0-9]* | wc -l,
if that fails, depending on the config, it either tries to count CPUs
in /proc/cpuinfo, or returns the _SC_NPROCESSORS_ONLN value instead.
If /proc/cpuinfo has some issue, it returns just 1 worst case. This
oddity is nothing new [1], but semantics/behaviour seems to be settled.
_SC_NPROCESSORS_ONLN will parse /sys/devices/system/cpu/online, if
that fails it looks into /proc/stat for cpuX entries, and if also that
fails for some reason, /proc/cpuinfo is consulted (and returning 1 if
unlikely all breaks down).
While that might match num_possible_cpus() from the kernel in some
cases, it's really not guaranteed with CPU hotplugging, and can result
in a buffer overflow since the array in user space could have too few
number of slots, and on perpcu map lookup, the kernel will write beyond
that memory of the value buffer.
William Tu reported such mismatches:
[...] The fact that sysconf(_SC_NPROCESSORS_CONF) != num_possible_cpu()
happens when CPU hotadd is enabled. For example, in Fusion when
setting vcpu.hotadd = "TRUE" or in KVM, setting ./qemu-system-x86_64
-smp 2, maxcpus=4 ... the num_possible_cpu() will be 4 and sysconf()
will be 2 [2]. [...]
Documentation/cputopology.txt says /sys/devices/system/cpu/possible
outputs cpu_possible_mask. That is the same as in num_possible_cpus(),
so first step would be to fix the _SC_NPROCESSORS_CONF calls with our
own implementation. Later, we could add support to bpf(2) for passing
a mask via CPU_SET(3), for example, to just select a subset of CPUs.
BPF samples code needs this fix as well (at least so that people stop
copying this). Thus, define bpf_num_possible_cpus() once in selftests
and import it from there for the sample code to avoid duplicating it.
The remaining sysconf(_SC_NPROCESSORS_CONF) in samples are unrelated.
Thanks,
Daniel
WRITE_ONCE(cpus, tmp_cpus);
return tmp_cpus;
