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tracing: Do not allocate buffer in trace_find_next_entry() in atomic

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When dumping out the trace data in latency format, a check is made to peek
at the next event to compare its timestamp to the current one, and if the
delta is of a greater size, it will add a marker showing so. But to do this,
it needs to save the current event otherwise peeking at the next event will
remove the current event. To save the event, a temp buffer is used, and if
the event is bigger than the temp buffer, the temp buffer is freed and a
bigger buffer is allocated.

This allocation is a problem when called in atomic context. The only way
this gets called via atomic context is via ftrace_dump(). Thus, use a static
buffer of 128 bytes (which covers most events), and if the event is bigger
than that, simply return NULL. The callers of trace_find_next_entry() need
to handle a NULL case, as that's what would happen if the allocation failed.

Link: https://lore.kernel.org/r/20200326091256.GR11705@shao2-debian

Fixes: ff895103a8 ("tracing: Save off entry when peeking at next entry")
Reported-by: kernel test robot <rong.a.chen@intel.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
This commit is contained in:
Steven Rostedt (VMware) 2020-04-01 22:44:46 -04:00
parent 2ab2a0924b
commit 8e99cf91b9
1 changed files with 20 additions and 1 deletions
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21
kernel/trace/trace.c
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@ -3472,6 +3472,9 @@ __find_next_entry(struct trace_iterator *iter, int *ent_cpu,
return next;
}
#define STATIC_TEMP_BUF_SIZE 128
static char static_temp_buf[STATIC_TEMP_BUF_SIZE];
/* Find the next real entry, without updating the iterator itself */
struct trace_entry *trace_find_next_entry(struct trace_iterator *iter,
int *ent_cpu, u64 *ent_ts)
@ -3480,13 +3483,26 @@ struct trace_entry *trace_find_next_entry(struct trace_iterator *iter,
int ent_size = iter->ent_size;
struct trace_entry *entry;
/*
* If called from ftrace_dump(), then the iter->temp buffer
* will be the static_temp_buf and not created from kmalloc.
* If the entry size is greater than the buffer, we can
* not save it. Just return NULL in that case. This is only
* used to add markers when two consecutive events' time
* stamps have a large delta. See trace_print_lat_context()
*/
if (iter->temp == static_temp_buf &&
STATIC_TEMP_BUF_SIZE < ent_size)
return NULL;
/*
* The __find_next_entry() may call peek_next_entry(), which may
* call ring_buffer_peek() that may make the contents of iter->ent
* undefined. Need to copy iter->ent now.
*/
if (iter->ent && iter->ent != iter->temp) {
if (!iter->temp || iter->temp_size < iter->ent_size) {
if ((!iter->temp || iter->temp_size < iter->ent_size) &&
!WARN_ON_ONCE(iter->temp == static_temp_buf)) {
kfree(iter->temp);
iter->temp = kmalloc(iter->ent_size, GFP_KERNEL);
if (!iter->temp)
@ -9203,6 +9219,9 @@ void ftrace_dump(enum ftrace_dump_mode oops_dump_mode)
/* Simulate the iterator */
trace_init_global_iter(&iter);
/* Can not use kmalloc for iter.temp */
iter.temp = static_temp_buf;
iter.temp_size = STATIC_TEMP_BUF_SIZE;
for_each_tracing_cpu(cpu) {
atomic_inc(&per_cpu_ptr(iter.array_buffer->data, cpu)->disabled);