linux-kernel - Re: perf/jit doesn't cope well with mprotect() to jit containing pages

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Date:   Thu, 26 Jan 2017 23:15:08 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andres Freund <andres@...razel.de>
Cc:     linux-kernel@...r.kernel.org,
        Stephane Eranian <eranian@...gle.com>, acme@...nel.org,
        jolsa@...hat.com, mingo@...e.hu, anton@...abs.org,
        namhyung@...nel.org, Stephane Eranian <eranian@...il.com>
Subject: Re: perf/jit doesn't cope well with mprotect() to jit containing
 pages

On Mon, Dec 12, 2016 at 09:49:03AM +0100, Peter Zijlstra wrote:

> > BTW, it's also a bit weird that those MMAP2 records triggered by
> > mprotect/mmap, have prot set to 0...
> 
> Yes, mprotect() does: vma->vm_flags = newflags; before calling
> perf_event_mmap(vma); which then looks at VM_{READ,WRITE,EXEC} bits in
> that word to generate the prot value.
> 
> So that being 0 is a bit weird.

OK, that was a silly bug :/

With that fixed, the following proglet:

#include <unistd.h>
#include <signal.h>
#include <stdio.h>
#include <malloc.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/mman.h>

void main(void)
{
	void *ptr[5];
	int i;

	for (i=0; i<5; i++) {
		ptr[i] = mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
		*(char *)ptr[i] = '\0';
		mprotect(ptr[i], 4096, PROT_EXEC);
	}
}


Gives:

root@...-ep:~# perf record -d ./mprot
...
root@...-ep:~# perf report -D | grep MMAP2
...
43805378481 0xc60 [0x60]: PERF_RECORD_MMAP2 2073/2073: [0x7f6ec0862000(0x1000) @ 0x7f6ec0862000 00:00 0 0]: --xp //anon
43805381755 0xcc0 [0x60]: PERF_RECORD_MMAP2 2073/2073: [0x7f6ec0861000(0x1000) @ 0x7f6ec0861000 00:00 0 0]: rw-p //anon
43805392374 0xd20 [0x60]: PERF_RECORD_MMAP2 2073/2073: [0x7f6ec0861000(0x2000) @ 0x7f6ec0861000 00:00 0 0]: --xp //anon
43805395093 0xd80 [0x60]: PERF_RECORD_MMAP2 2073/2073: [0x7f6ec0860000(0x1000) @ 0x7f6ec0860000 00:00 0 0]: rw-p //anon
43805402842 0xde0 [0x60]: PERF_RECORD_MMAP2 2073/2073: [0x7f6ec0860000(0x3000) @ 0x7f6ec0860000 00:00 0 0]: --xp //anon
43805405388 0xe40 [0x60]: PERF_RECORD_MMAP2 2073/2073: [0x7f6ec085f000(0x1000) @ 0x7f6ec085f000 00:00 0 0]: rw-p //anon
43805412605 0xea0 [0x60]: PERF_RECORD_MMAP2 2073/2073: [0x7f6ec085f000(0x4000) @ 0x7f6ec085f000 00:00 0 0]: --xp //anon
43805415134 0xf00 [0x60]: PERF_RECORD_MMAP2 2073/2073: [0x7f6ec085e000(0x1000) @ 0x7f6ec085e000 00:00 0 0]: rw-p //anon
43805422225 0xf60 [0x60]: PERF_RECORD_MMAP2 2073/2073: [0x7f6ec085e000(0x5000) @ 0x7f6ec085e000 00:00 0 0]: --xp //anon

If you omit the mmap_data=1 thing, it looks like:

123087941779 0x500 [0x60]: PERF_RECORD_MMAP2 2084/2084: [0x7fca5a8bd000(0x1000) @ 0x7fca5a8bd000 00:00 0 0]: --xp //anon
123087953825 0x560 [0x60]: PERF_RECORD_MMAP2 2084/2084: [0x7fca5a8bc000(0x2000) @ 0x7fca5a8bc000 00:00 0 0]: --xp //anon
123087962944 0x5c0 [0x60]: PERF_RECORD_MMAP2 2084/2084: [0x7fca5a8bb000(0x3000) @ 0x7fca5a8bb000 00:00 0 0]: --xp //anon
123087971135 0x620 [0x60]: PERF_RECORD_MMAP2 2084/2084: [0x7fca5a8ba000(0x4000) @ 0x7fca5a8ba000 00:00 0 0]: --xp //anon
123087979360 0x680 [0x60]: PERF_RECORD_MMAP2 2084/2084: [0x7fca5a8b9000(0x5000) @ 0x7fca5a8b9000 00:00 0 0]: --xp //anon

---
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 896aa53..e67c5ba 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -6660,6 +6683,27 @@ static void perf_event_mmap_event(struct perf_mmap_event *mmap_event)
 	char *buf = NULL;
 	char *name;
 
+	if (vma->vm_flags & VM_READ)
+		prot |= PROT_READ;
+	if (vma->vm_flags & VM_WRITE)
+		prot |= PROT_WRITE;
+	if (vma->vm_flags & VM_EXEC)
+		prot |= PROT_EXEC;
+
+	if (vma->vm_flags & VM_MAYSHARE)
+		flags = MAP_SHARED;
+	else
+		flags = MAP_PRIVATE;
+
+	if (vma->vm_flags & VM_DENYWRITE)
+		flags |= MAP_DENYWRITE;
+	if (vma->vm_flags & VM_MAYEXEC)
+		flags |= MAP_EXECUTABLE;
+	if (vma->vm_flags & VM_LOCKED)
+		flags |= MAP_LOCKED;
+	if (vma->vm_flags & VM_HUGETLB)
+		flags |= MAP_HUGETLB;
+
 	if (file) {
 		struct inode *inode;
 		dev_t dev;
@@ -6686,27 +6730,6 @@ static void perf_event_mmap_event(struct perf_mmap_event *mmap_event)
 		maj = MAJOR(dev);
 		min = MINOR(dev);
 
-		if (vma->vm_flags & VM_READ)
-			prot |= PROT_READ;
-		if (vma->vm_flags & VM_WRITE)
-			prot |= PROT_WRITE;
-		if (vma->vm_flags & VM_EXEC)
-			prot |= PROT_EXEC;
-
-		if (vma->vm_flags & VM_MAYSHARE)
-			flags = MAP_SHARED;
-		else
-			flags = MAP_PRIVATE;
-
-		if (vma->vm_flags & VM_DENYWRITE)
-			flags |= MAP_DENYWRITE;
-		if (vma->vm_flags & VM_MAYEXEC)
-			flags |= MAP_EXECUTABLE;
-		if (vma->vm_flags & VM_LOCKED)
-			flags |= MAP_LOCKED;
-		if (vma->vm_flags & VM_HUGETLB)
-			flags |= MAP_HUGETLB;
-
 		goto got_name;
 	} else {
 		if (vma->vm_ops && vma->vm_ops->name) {