On 2/2/24 4:21 PM, Siddharth Chintamaneni wrote:
On Tue, 30 Jan 2024 at 04:25, Jiri Olsa <olsajiri@xxxxxxxxx> wrote:
On Wed, Jan 24, 2024 at 10:43:32AM -0500, Siddharth Chintamaneni wrote:
While we were working on some experiments with BPF trampoline, we came
across a deadlock scenario that could happen.
A deadlock happens when two nested BPF programs tries to acquire the
same lock i.e, If a BPF program is attached using fexit to
bpf_spin_lock or using a fentry to bpf_spin_unlock, and it then
attempts to acquire the same lock as the previous BPF program, a
deadlock situation arises.
Here is an example:
SEC(fentry/bpf_spin_unlock)
int fentry_2{
bpf_spin_lock(&x->lock);
bpf_spin_unlock(&x->lock);
}
SEC(fentry/xxx)
int fentry_1{
bpf_spin_lock(&x->lock);
bpf_spin_unlock(&x->lock);
}
hi,
looks like valid issue, could you add selftest for that?
Hello,
I have added selftest for the deadlock scenario.
I wonder we could restrict just programs that use bpf_spin_lock/bpf_spin_unlock
helpers? I'm not sure there's any useful use case for tracing spin lock helpers,
but I think we should at least try this before we deny it completely
If we restrict programs (attached to spinlock helpers) that use
bpf_spin_lock/unlock helpers, there could be a scenario where a helper
function called within the program has a BPF program attached that
tries to acquire the same lock.
To prevent these cases, a simple fix could be adding these helpers to
denylist in the verifier. This fix will prevent the BPF programs from
being loaded by the verifier.
previously, a similar solution was proposed to prevent recursion.
https://lore.kernel.org/lkml/20230417154737.12740-2-laoar.shao@xxxxxxxxx/
the difference is that __rcu_read_lock/__rcu_read_unlock are called unconditionally
(always) when executing bpf tracing probe, the problem you described above is only
for programs calling spin lock helpers (on same spin lock)
Signed-off-by: Siddharth Chintamaneni <sidchintamaneni@xxxxxx>
---
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 65f598694d55..8f1834f27f81 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -20617,6 +20617,10 @@ BTF_ID(func, preempt_count_sub)
BTF_ID(func, __rcu_read_lock)
BTF_ID(func, __rcu_read_unlock)
#endif
+#if defined(CONFIG_DYNAMIC_FTRACE)
why the CONFIG_DYNAMIC_FTRACE dependency?
As we described in the self-tests, nesting of multiple BPF programs
could only happen with fentry/fexit programs when DYNAMIC_FTRACE is
enabled. In other scenarios, when DYNAMIC_FTRACE is disabled, a BPF
program cannot be attached to any helper functions.
jirka
+BTF_ID(func, bpf_spin_lock)
+BTF_ID(func, bpf_spin_unlock)
+#endif
BTF_SET_END(btf_id_deny)
Currently, we already have 'notrace' marked to bpf_spin_lock
and bpf_spin_unlock:
notrace BPF_CALL_1(bpf_spin_lock, struct bpf_spin_lock *, lock)
{
__bpf_spin_lock_irqsave(lock);
return 0;
}
notrace BPF_CALL_1(bpf_spin_unlock, struct bpf_spin_lock *, lock)
{
__bpf_spin_unlock_irqrestore(lock);
return 0;
}
But unfortunately, BPF_CALL_* macros put notrace to the static
inline function ____bpf_spin_lock()/____bpf_spin_unlock(), and not
to static function bpf_spin_lock()/bpf_spin_unlock().
I think the following is a better fix and reflects original
intention:
diff --git a/include/linux/filter.h b/include/linux/filter.h
index fee070b9826e..779f8ee71607 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -566,6 +566,25 @@ static inline bool insn_is_zext(const struct bpf_insn *insn)
#define BPF_CALL_4(name, ...) BPF_CALL_x(4, name, __VA_ARGS__)
#define BPF_CALL_5(name, ...) BPF_CALL_x(5, name, __VA_ARGS__)
+#define NOTRACE_BPF_CALL_x(x, name, ...) \
+ static __always_inline \
+ u64 ____##name(__BPF_MAP(x, __BPF_DECL_ARGS, __BPF_V, __VA_ARGS__)); \
+ typedef u64 (*btf_##name)(__BPF_MAP(x, __BPF_DECL_ARGS, __BPF_V, __VA_ARGS__)); \
+ notrace u64 name(__BPF_REG(x, __BPF_DECL_REGS, __BPF_N, __VA_ARGS__)); \
+ notrace u64 name(__BPF_REG(x, __BPF_DECL_REGS, __BPF_N, __VA_ARGS__)) \
+ { \
+ return ((btf_##name)____##name)(__BPF_MAP(x,__BPF_CAST,__BPF_N,__VA_ARGS__));\
+ } \
+ static __always_inline \
+ u64 ____##name(__BPF_MAP(x, __BPF_DECL_ARGS, __BPF_V, __VA_ARGS__))
+
+#define NOTRACE_BPF_CALL_0(name, ...) NOTRACE_BPF_CALL_x(0, name, __VA_ARGS__)
+#define NOTRACE_BPF_CALL_1(name, ...) NOTRACE_BPF_CALL_x(1, name, __VA_ARGS__)
+#define NOTRACE_BPF_CALL_2(name, ...) NOTRACE_BPF_CALL_x(2, name, __VA_ARGS__)
+#define NOTRACE_BPF_CALL_3(name, ...) NOTRACE_BPF_CALL_x(3, name, __VA_ARGS__)
+#define NOTRACE_BPF_CALL_4(name, ...) NOTRACE_BPF_CALL_x(4, name, __VA_ARGS__)
+#define NOTRACE_BPF_CALL_5(name, ...) NOTRACE_BPF_CALL_x(5, name, __VA_ARGS__)
+
#define bpf_ctx_range(TYPE, MEMBER) \
offsetof(TYPE, MEMBER) ... offsetofend(TYPE, MEMBER) - 1
#define bpf_ctx_range_till(TYPE, MEMBER1, MEMBER2) \
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 4db1c658254c..87136e27a99a 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -334,7 +334,7 @@ static inline void __bpf_spin_lock_irqsave(struct bpf_spin_lock *lock)
__this_cpu_write(irqsave_flags, flags);
}
-notrace BPF_CALL_1(bpf_spin_lock, struct bpf_spin_lock *, lock)
+NOTRACE_BPF_CALL_1(bpf_spin_lock, struct bpf_spin_lock *, lock)
{
__bpf_spin_lock_irqsave(lock);
return 0;
@@ -357,7 +357,7 @@ static inline void __bpf_spin_unlock_irqrestore(struct bpf_spin_lock *lock)
local_irq_restore(flags);
}
-notrace BPF_CALL_1(bpf_spin_unlock, struct bpf_spin_lock *, lock)
+NOTRACE_BPF_CALL_1(bpf_spin_unlock, struct bpf_spin_lock *, lock)
{
__bpf_spin_unlock_irqrestore(lock);
return 0;
Macros NOTRACE_BPF_CALL_*() could be consolated with BPF_CALL_*() but I think
a separate macro might be easier to understand.
Signed-off-by: Siddharth Chintamaneni <sidchintamaneni@xxxxxx>
---
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 65f598694d55..ffc2515195f1 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -20617,6 +20617,10 @@ BTF_ID(func, preempt_count_sub)
BTF_ID(func, __rcu_read_lock)
BTF_ID(func, __rcu_read_unlock)
#endif
+#ifdef CONFIG_DYNAMIC_FTRACE
+BTF_ID(func, bpf_spin_lock)
+BTF_ID(func, bpf_spin_unlock)
+#endif
BTF_SET_END(btf_id_deny)
static bool can_be_sleepable(struct bpf_prog *prog)
[...]
