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bpf: Provide recursion prevention helpers

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The places which need to prevent the execution of trace type BPF programs
to prevent deadlocks on the hash bucket lock do this open coded.

Provide two inline functions, bpf_disable/enable_instrumentation() to
replace these open coded protection constructs.

Use migrate_disable/enable() instead of preempt_disable/enable() right away
so this works on RT enabled kernels. On a !RT kernel migrate_disable /
enable() are mapped to preempt_disable/enable().

These helpers use this_cpu_inc/dec() instead of __this_cpu_inc/dec() on an
RT enabled kernel because migrate disabled regions are preemptible and
preemption might hit in the middle of a RMW operation which can lead to
inconsistent state.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200224145644.103910133@linutronix.de
This commit is contained in:
Thomas Gleixner 2020-02-24 15:01:47 +01:00 committed by Alexei Starovoitov
parent 2a916f2f54
commit c518cfa0c5
1 changed files with 30 additions and 0 deletions
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30
include/linux/bpf.h
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@ -961,6 +961,36 @@ _out: \
#ifdef CONFIG_BPF_SYSCALL
DECLARE_PER_CPU(int, bpf_prog_active);
/*
* Block execution of BPF programs attached to instrumentation (perf,
* kprobes, tracepoints) to prevent deadlocks on map operations as any of
* these events can happen inside a region which holds a map bucket lock
* and can deadlock on it.
*
* Use the preemption safe inc/dec variants on RT because migrate disable
* is preemptible on RT and preemption in the middle of the RMW operation
* might lead to inconsistent state. Use the raw variants for non RT
* kernels as migrate_disable() maps to preempt_disable() so the slightly
* more expensive save operation can be avoided.
*/
static inline void bpf_disable_instrumentation(void)
{
migrate_disable();
if (IS_ENABLED(CONFIG_PREEMPT_RT))
this_cpu_inc(bpf_prog_active);
else
__this_cpu_inc(bpf_prog_active);
}
static inline void bpf_enable_instrumentation(void)
{
if (IS_ENABLED(CONFIG_PREEMPT_RT))
this_cpu_dec(bpf_prog_active);
else
__this_cpu_dec(bpf_prog_active);
migrate_enable();
}
extern const struct file_operations bpf_map_fops;
extern const struct file_operations bpf_prog_fops;