Messages in this thread Patch in this message | | | From | Florent Revest <> | Subject | [PATCH bpf] bpf: Don't WARN_ON_ONCE in bpf_bprintf_prepare | Date | Wed, 5 May 2021 18:23:07 +0200 |
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The bpf_seq_printf, bpf_trace_printk and bpf_snprintf helpers share one per-cpu buffer that they use to store temporary data (arguments to bprintf). They "get" that buffer with try_get_fmt_tmp_buf and "put" it by the end of their scope with bpf_bprintf_cleanup.
If one of these helpers gets called within the scope of one of these helpers, for example: a first bpf program gets called, uses bpf_trace_printk which calls raw_spin_lock_irqsave which is traced by another bpf program that calls bpf_trace_printk again, then the second "get" fails. Essentially, these helpers are not re-entrant, and it's not that bad because they would simply return -EBUSY and recover gracefully.
However, when this happens, the code hits a WARN_ON_ONCE. The guidelines in include/asm-generic/bug.h say "Do not use these macros [...] on transient conditions like ENOMEM or EAGAIN."
This condition qualifies as transient, for example, the next raw_spin_lock_irqsave probe is likely to succeed, so it does not deserve a WARN_ON_ONCE.
The guidelines also say "Do not use these macros when checking for invalid external inputs (e.g. invalid system call arguments" and, in a way, this can be seen as an invalid input because syzkaller triggered it.
Signed-off-by: Florent Revest <revest@chromium.org> Reported-by: syzbot <syzbot@syzkaller.appspotmail.com> Fixes: d9c9e4db186a ("bpf: Factorize bpf_trace_printk and bpf_seq_printf") --- kernel/bpf/helpers.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c index 544773970dbc..007fa26eb3f5 100644 --- a/kernel/bpf/helpers.c +++ b/kernel/bpf/helpers.c @@ -709,7 +709,7 @@ static int try_get_fmt_tmp_buf(char **tmp_buf) preempt_disable(); used = this_cpu_inc_return(bpf_printf_buf_used); - if (WARN_ON_ONCE(used > 1)) { + if (used > 1) { this_cpu_dec(bpf_printf_buf_used); preempt_enable(); return -EBUSY; -- 2.31.1.527.g47e6f16901-goog
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