[PATCH 4.12 71/99] ring-buffer: Have ring_buffer_alloc_read_page() return error on offline CPU

Greg Kroah-Hartman
2017-08-28T09:08:51Z

From: Steven Rostedt (VMware) <rostedt@goodmis.org>

commit a7e52ad7ed82e21273eccff93d1477a7b313aabb upstream.

Chunyu Hu reported:
"per_cpu trace directories and files are created for all possible cpus,
but only the cpus which have ever been on-lined have their own per cpu
ring buffer (allocated by cpuhp threads). While trace_buffers_open, the
open handler for trace file 'trace_pipe_raw' is always trying to access
field of ring_buffer_per_cpu, and would panic with the NULL pointer.

Align the behavior of trace_pipe_raw with trace_pipe, that returns -NODEV
when openning it if that cpu does not have trace ring buffer.

Reproduce:
cat /sys/kernel/debug/tracing/per_cpu/cpu31/trace_pipe_raw
(cpu31 is never on-lined, this is a 16 cores x86_64 box)

Tested with:
1) boot with maxcpus=14, read trace_pipe_raw of cpu15.
Got -NODEV.
2) oneline cpu15, read trace_pipe_raw of cpu15.
Get the raw trace data.

Call trace:
[ 5760.950995] RIP: 0010:ring_buffer_alloc_read_page+0x32/0xe0
[ 5760.961678] tracing_buffers_read+0x1f6/0x230
[ 5760.962695] __vfs_read+0x37/0x160
[ 5760.963498] ? __vfs_read+0x5/0x160
[ 5760.964339] ? security_file_permission+0x9d/0xc0
[ 5760.965451] ? __vfs_read+0x5/0x160
[ 5760.966280] vfs_read+0x8c/0x130
[ 5760.967070] SyS_read+0x55/0xc0
[ 5760.967779] do_syscall_64+0x67/0x150
[ 5760.968687] entry_SYSCALL64_slow_path+0x25/0x25"

This was introduced by the addition of the feature to reuse reader pages
instead of re-allocating them. The problem is that the allocation of a
reader page (which is per cpu) does not check if the cpu is online and set
up for the ring buffer.

Link: http://lkml.kernel.org/r/1500880866-1177-1-git-send-email-chuhu@redhat.com

Fixes: 73a757e63114 ("ring-buffer: Return reader page back into existing ring buffer")
Reported-by: Chunyu Hu <chuhu@redhat.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
kernel/trace/ring_buffer.c | 14 +++++++++-----
kernel/trace/ring_buffer_benchmark.c | 2 +-
kernel/trace/trace.c | 16 +++++++++++-----
3 files changed, 21 insertions(+), 11 deletions(-)

--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -4386,15 +4386,19 @@ EXPORT_SYMBOL_GPL(ring_buffer_swap_cpu);
* the page that was allocated, with the read page of the buffer.
*
* Returns:
- * The page allocated, or NULL on error.
+ * The page allocated, or ERR_PTR
*/
void *ring_buffer_alloc_read_page(struct ring_buffer *buffer, int cpu)
{
- struct ring_buffer_per_cpu *cpu_buffer = buffer->buffers[cpu];
+ struct ring_buffer_per_cpu *cpu_buffer;
struct buffer_data_page *bpage = NULL;
unsigned long flags;
struct page *page;

+ if (!cpumask_test_cpu(cpu, buffer->cpumask))
+ return ERR_PTR(-ENODEV);
+
+ cpu_buffer = buffer->buffers[cpu];
local_irq_save(flags);
arch_spin_lock(&cpu_buffer->lock);

@@ -4412,7 +4416,7 @@ void *ring_buffer_alloc_read_page(struct
page = alloc_pages_node(cpu_to_node(cpu),
GFP_KERNEL | __GFP_NORETRY, 0);
if (!page)
- return NULL;
+ return ERR_PTR(-ENOMEM);

bpage = page_address(page);

@@ -4467,8 +4471,8 @@ EXPORT_SYMBOL_GPL(ring_buffer_free_read_
*
* for example:
* rpage = ring_buffer_alloc_read_page(buffer, cpu);
- * if (!rpage)
- * return error;
+ * if (IS_ERR(rpage))
+ * return PTR_ERR(rpage);
* ret = ring_buffer_read_page(buffer, &rpage, len, cpu, 0);
* if (ret >= 0)
* process_page(rpage, ret);
--- a/kernel/trace/ring_buffer_benchmark.c
+++ b/kernel/trace/ring_buffer_benchmark.c
@@ -113,7 +113,7 @@ static enum event_status read_page(int c
int i;

bpage = ring_buffer_alloc_read_page(buffer, cpu);
- if (!bpage)
+ if (IS_ERR(bpage))
return EVENT_DROPPED;

ret = ring_buffer_read_page(buffer, &bpage, PAGE_SIZE, cpu, 1);
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -6403,7 +6403,7 @@ tracing_buffers_read(struct file *filp,
{
struct ftrace_buffer_info *info = filp->private_data;
struct trace_iterator *iter = &info->iter;
- ssize_t ret;
+ ssize_t ret = 0;
ssize_t size;

if (!count)
@@ -6417,10 +6417,15 @@ tracing_buffers_read(struct file *filp,
if (!info->spare) {
info->spare = ring_buffer_alloc_read_page(iter->trace_buffer->buffer,
iter->cpu_file);
- info->spare_cpu = iter->cpu_file;
+ if (IS_ERR(info->spare)) {
+ ret = PTR_ERR(info->spare);
+ info->spare = NULL;
+ } else {
+ info->spare_cpu = iter->cpu_file;
+ }
}
if (!info->spare)
- return -ENOMEM;
+ return ret;

/* Do we have previous read data to read? */
if (info->read < PAGE_SIZE)
@@ -6595,8 +6600,9 @@ tracing_buffers_splice_read(struct file
ref->ref = 1;
ref->buffer = iter->trace_buffer->buffer;
ref->page = ring_buffer_alloc_read_page(ref->buffer, iter->cpu_file);
- if (!ref->page) {
- ret = -ENOMEM;
+ if (IS_ERR(ref->page)) {
+ ret = PTR_ERR(ref->page);
+ ref->page = NULL;
kfree(ref);
break;
}