Re: [regression] tracing: kmemleak warning in allocate_cmdlines_buffer()

[Steven Rostedt]

On Wed, 14 Feb 2024 14:50:56 +0200
Kalle Valo <kvalo@xxxxxxxxxx> wrote:

> Hi Steven,
> 
> I upgraded our ath11k test setup to v6.8-rc4 and noticed a new kmemleak
> warning in the log:

Thanks for the report.

> 
> unreferenced object 0xffff8881010c8000 (size 32760):
>   comm "swapper", pid 0, jiffies 4294667296
>   hex dump (first 32 bytes):
>     ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff  ................
>     ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff  ................
>   backtrace (crc ae6ec1b9):
>     [<ffffffff86722405>] kmemleak_alloc+0x45/0x80
>     [<ffffffff8414028d>] __kmalloc_large_node+0x10d/0x190
>     [<ffffffff84146ab1>] __kmalloc+0x3b1/0x4c0
>     [<ffffffff83ed7103>] allocate_cmdlines_buffer+0x113/0x230
>     [<ffffffff88649c34>] tracer_alloc_buffers.isra.0+0x124/0x460
>     [<ffffffff8864a174>] early_trace_init+0x14/0xa0
>     [<ffffffff885dd5ae>] start_kernel+0x12e/0x3c0
>     [<ffffffff885f5758>] x86_64_start_reservations+0x18/0x30
>     [<ffffffff885f582b>] x86_64_start_kernel+0x7b/0x80
>     [<ffffffff83a001c3>] secondary_startup_64_no_verify+0x15e/0x16b
> 
> I don't see this warning in v6.8-rc3 and also reverting commit
> 44dc5c41b5b1 ("tracing: Fix wasted memory in saved_cmdlines logic")
> makes the warning go away. Let me know if you need more info or help
> with testing, I see the warning every time so it's easy to reproduce.
> 

Hmm, I changed the code a bit and I wonder if this is a false positive?

Instead of allocating the structure via kmalloc() I now use it as part of a
page.

That is, the old code had:

        s = kmalloc(sizeof(*s), GFP_KERNEL);
	s->saved_cmdlines = kmalloc_array(TASK_COMM_LEN, val, GFP_KERNEL);

Where as the new code has:

	orig_size = sizeof(*s) + val * TASK_COMM_LEN;
	order = get_order(orig_size);
	size = 1 << (order + PAGE_SHIFT);
	page = alloc_pages(GFP_KERNEL, order);
	if (!page)
		return NULL;

	s = page_address(page);
	memset(s, 0, sizeof(*s));

	s->saved_cmdlines = kmalloc_array(TASK_COMM_LEN, val, GFP_KERNEL);


Does kmemleak handle structures that are assigned to alloc_pages()
allocations? I don't think it does.

I think we need to inform kmemleak about this. Does the following patch fix
this for you?

-- Steve

diff --git a/kernel/trace/trace_sched_switch.c b/kernel/trace/trace_sched_switch.c
index e4fbcc3bede5..de4182224ea2 100644
--- a/kernel/trace/trace_sched_switch.c
+++ b/kernel/trace/trace_sched_switch.c
@@ -9,6 +9,7 @@
 #include <linux/kallsyms.h>
 #include <linux/uaccess.h>
 #include <linux/ftrace.h>
+#include <linux/kmemleak.h>
 #include <trace/events/sched.h>
 
 #include "trace.h"
@@ -190,6 +191,7 @@ static void free_saved_cmdlines_buffer(struct saved_cmdlines_buffer *s)
 	int order = get_order(sizeof(*s) + s->cmdline_num * TASK_COMM_LEN);
 
 	kfree(s->map_cmdline_to_pid);
+	kmemleak_free(s);
 	free_pages((unsigned long)s, order);
 }
 
@@ -210,6 +212,7 @@ static struct saved_cmdlines_buffer *allocate_cmdlines_buffer(unsigned int val)
 
 	s = page_address(page);
 	memset(s, 0, sizeof(*s));
+	kmemleak_alloc(s, size, 1, GFP_KERNEL);
 
 	/* Round up to actual allocation */
 	val = (size - sizeof(*s)) / TASK_COMM_LEN;