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bpf: enable stackmap with build_id in nmi context 

Currently, we cannot parse build_id in nmi context because of
up_read(&current->mm->mmap_sem), this makes stackmap with build_id
less useful. This patch enables parsing build_id in nmi by putting
the up_read() call in irq_work. To avoid memory allocation in nmi
context, we use per cpu variable for the irq_work. As a result, only
one irq_work per cpu is allowed. If the irq_work is in-use, we
fallback to only report ips.

Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
This commit is contained in:
Song Liu 2018-05-07 10:50:48 -07:00 committed by Daniel Borkmann
parent a84880ef43
commit bae77c5eb5
2 changed files with 54 additions and 6 deletions
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1
init/Kconfig
View File

@ -1391,6 +1391,7 @@ config BPF_SYSCALL
bool "Enable bpf() system call"
select ANON_INODES
select BPF
select IRQ_WORK
default n
help
Enable the bpf() system call that allows to manipulate eBPF

59
kernel/bpf/stackmap.c
View File

@ -11,6 +11,7 @@
#include <linux/perf_event.h>
#include <linux/elf.h>
#include <linux/pagemap.h>
#include <linux/irq_work.h>
#include "percpu_freelist.h"
#define STACK_CREATE_FLAG_MASK \
@ -32,6 +33,23 @@ struct bpf_stack_map {
struct stack_map_bucket *buckets[];
};
/* irq_work to run up_read() for build_id lookup in nmi context */
struct stack_map_irq_work {
struct irq_work irq_work;
struct rw_semaphore *sem;
};
static void do_up_read(struct irq_work *entry)
{
struct stack_map_irq_work *work;
work = container_of(entry, struct stack_map_irq_work, irq_work);
up_read(work->sem);
work->sem = NULL;
}
static DEFINE_PER_CPU(struct stack_map_irq_work, up_read_work);
static inline bool stack_map_use_build_id(struct bpf_map *map)
{
return (map->map_flags & BPF_F_STACK_BUILD_ID);
@ -267,17 +285,27 @@ static void stack_map_get_build_id_offset(struct bpf_stack_build_id *id_offs,
{
int i;
struct vm_area_struct *vma;
bool in_nmi_ctx = in_nmi();
bool irq_work_busy = false;
struct stack_map_irq_work *work;
if (in_nmi_ctx) {
work = this_cpu_ptr(&up_read_work);
if (work->irq_work.flags & IRQ_WORK_BUSY)
/* cannot queue more up_read, fallback */
irq_work_busy = true;
}
/*
* We cannot do up_read() in nmi context, so build_id lookup is
* only supported for non-nmi events. If at some point, it is
* possible to run find_vma() without taking the semaphore, we
* would like to allow build_id lookup in nmi context.
* We cannot do up_read() in nmi context. To do build_id lookup
* in nmi context, we need to run up_read() in irq_work. We use
* a percpu variable to do the irq_work. If the irq_work is
* already used by another lookup, we fall back to report ips.
*
* Same fallback is used for kernel stack (!user) on a stackmap
* with build_id.
*/
if (!user || !current || !current->mm || in_nmi() ||
if (!user || !current || !current->mm || irq_work_busy ||
down_read_trylock(&current->mm->mmap_sem) == 0) {
/* cannot access current->mm, fall back to ips */
for (i = 0; i < trace_nr; i++) {
@ -299,7 +327,13 @@ static void stack_map_get_build_id_offset(struct bpf_stack_build_id *id_offs,
- vma->vm_start;
id_offs[i].status = BPF_STACK_BUILD_ID_VALID;
}
up_read(&current->mm->mmap_sem);
if (!in_nmi_ctx) {
up_read(&current->mm->mmap_sem);
} else {
work->sem = &current->mm->mmap_sem;
irq_work_queue(&work->irq_work);
}
}
BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, struct bpf_map *, map,
@ -575,3 +609,16 @@ const struct bpf_map_ops stack_map_ops = {
.map_update_elem = stack_map_update_elem,
.map_delete_elem = stack_map_delete_elem,
};
static int __init stack_map_init(void)
{
int cpu;
struct stack_map_irq_work *work;
for_each_possible_cpu(cpu) {
work = per_cpu_ptr(&up_read_work, cpu);
init_irq_work(&work->irq_work, do_up_read);
}
return 0;
}
subsys_initcall(stack_map_init);