Hi Adrian,

On Wed, Jun 09, 2021 at 11:23:25AM +0300, Adrian Hunter wrote:

[...]

quoted

quoted

I was thinking we would separate out the compat case:

#if BITS_PER_LONG == 32
	if (kernel_is_64_bit)
		return compat_auxtrace_mmap__[read_head/write_tail]()
#endif

So the non-compat cases would not be affected.

Because I don't want to introduce the complexity for read/write head
and tail, and we also need to handle the same issue for the perf ring
buffer.  So how about below change?

The main idea for below change is it allows the perf to run normally
on the compat mode and exitly if detects the buffer head is close to
the low 32-bit's overflow: when detect the low 32-bit value is bigger
than 0xf0000000 (so we have 256MiB margin to the overflow), it reports
error and exit.

diff --git a/tools/perf/util/auxtrace.c b/tools/perf/util/auxtrace.c
index 1b4091a3b508..2a9965bfeab4 100644
--- a/tools/perf/util/auxtrace.c
+++ b/tools/perf/util/auxtrace.c

@@ -1693,6 +1693,14 @@ static int __auxtrace_mmap__read(struct mmap *map,
 	pr_debug3("auxtrace idx %d old %#"PRIx64" head %#"PRIx64" diff %#"PRIx64"\n",
 		  mm->idx, old, head, head - old);
 
+#ifdef BITS_PER_LONG == 32
+	if (kernel_is_64bit() && head >= 0xf0000000) {

You are assuming the head never increases by more than 256MiB which
means you should limit the buffer size to 256MiB maximum.

To me this seems a bit too far from an ideal solution.

I would have thought separating out the compat case makes things
simpler to understand.

Agreed.  I will follow up the suggestions to add compat variants for
accessing AUX head and tail, and will distinguish compat case with
global env variable for 64-bit kernel.

After get ready, will send out for review. Thanks a lot for suggestions!

Leo

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