Re: [PATCH] tracing: hist: keep the entry count when the stats allocation fails

On Mon,  7 Sep 2026 15:03:23 +0900
Donggeun Yoo [off-list ref] wrote:

First I want to say thank you for all you fixes you have been sending. The
histogram code needs a lot more love that it has been given ;-)

Note, I've been changing the subject lines of your patches to:

This one:

 [PATCH] tracing: Keep the entrty count when the histogram stats allocation fails

And for you other patches:

 [PATCH] tracing: Free histogram ...

As histograms are not a separate subsystem and just part of the tracing
subsystem. And all subjects should start with a capital letter.

print_entries() uses n_entries both as the number of sort entries and as
its own return value, so the -ENOMEM it stores when the stats allocation
fails overwrites the count that the cleanup still needs:

	n_entries = tracing_map_sort_entries(map, ...);
	if (n_entries < 0)
		return n_entries;
	...
			if (!stats) {
				n_entries = -ENOMEM;
				goto out;
			}
	...
 out:
	tracing_map_destroy_sort_entries(sort_entries, n_entries);

tracing_map_destroy_sort_entries() takes an unsigned int and loops up to
it, so -ENOMEM arrives as 4294967284. It walks an array of at most
map->max_elts pointers and calls destroy_sort_entry(), which dereferences
and frees, on whatever lies past the end.

Reading the hist file of a trigger with a .percent value, with that
allocation forced to fail:

  BUG: KASAN: vmalloc-out-of-bounds in tracing_map_destroy_sort_entries+0xa0/0xb0
  Read of size 8 at addr ffffc90000045000 by task init/1
   tracing_map_destroy_sort_entries+0xa0/0xb0
   hist_show+0x6f7/0x1df0
   seq_read_iter+0x2b8/0x1190
   vfs_read+0x176/0xa40
  The buggy address belongs to a 4-page vmalloc region starting at
  ffffc90000041000 allocated at tracing_map_sort_entries+0x5c/0xd50

A few pages further the fault is fatal. The registers at the oops confirm
the bound: the loop's end pointer less the array start, over the pointer
size, is 4294967284.

Return the error in a separate variable and leave n_entries holding the
count, the way tracing_map_sort_entries() does on its own error path.

The stats block is only entered for a value carrying .percent or .graph,
which __create_val_field() has rejected since v6.3, so this cannot be
reached in mainline as it stands. It becomes reachable again with
"tracing: hist: let values keep the percent and graph modifiers", so it
should be applied first.

Fixes: abaa5258ce5e ("tracing: Add .percent suffix option to histogram values")
Cc: stable@vger.kernel.org

Please place the Cc stable above the fixes.

I'm not sure who is suggesting that but I'm seeing a lot of patches that do
that. To me, the Cc's (even to stable) should be the first thing in the
footer portion of the change log.

Signed-off-by: Donggeun Yoo <redacted>
---
Reproduced under QEMU x86_64 on 1fc5a74b108f, with the percent/graph patch
applied so the block is reachable and with the stats allocation forced to
fail. Before this change the read of the hist file never returns: KASAN
reports the vmalloc-out-of-bounds quoted above and the kernel then takes a
fatal page fault. After it the read returns, the histogram prints its
header with no rows, and KASAN is silent.

Link: https://lore.kernel.org/linux-trace-kernel/20260907052113.430818-1-donggeunyoo.kernel@gmail.com/ (local)

Informative links should go above the '---' and be included in the git
commit. In fact, you should always add a reported-by if something informed
you of a fix. I'll be adding to this patch:

Reported-by: sashiko-bot@kernel.org
Closes: https://lore.kernel.org/all/20260907053113.1CED91F00A3A@smtp.kernel.org/ (local)

-- Steve