Re: Re: BUG: Bad rss-counter state (4)

[Vegard Nossum]

(trimmed off the batman/bpf Ccs)

On 2020-05-18 14:28, syzbot wrote:
> syzbot has bisected this bug to:
>
> commit 0d8dd67be013727ae57645ecd3ea2c36365d7da8
> Author: Song Liu <songliubraving@xxxxxx>
> Date:   Wed Dec 6 22:45:14 2017 +0000
>
>      perf/headers: Sync new perf_event.h with the tools/include/uapi version
>
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=13240a02100000
> start commit:   ac935d22 Add linux-next specific files for 20200415
> git tree:       linux-next
> final crash:    https://syzkaller.appspot.com/x/report.txt?x=10a40a02100000
> console output: https://syzkaller.appspot.com/x/log.txt?x=17240a02100000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=bc498783097e9019
> dashboard link: https://syzkaller.appspot.com/bug?extid=347e2331d03d06ab0224
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=12d18e6e100000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=104170d6100000
>
> Reported-by: syzbot+347e2331d03d06ab0224@xxxxxxxxxxxxxxxxxxxxxxxxx
> Fixes: 0d8dd67be013 ("perf/headers: Sync new perf_event.h with the tools/include/uapi version")
>
> For information about bisection process see: https://goo.gl/tpsmEJ#bisection

FWIW here's a nicer reproducer that more clearly shows what's really
going on:

#define _GNU_SOURCE
#include <sys/mman.h>
#include <linux/perf_event.h>
#include <linux/hw_breakpoint.h>

#include <fcntl.h>
#include <stdio.h>
#include <stdint.h>
#include <string.h>
#include <syscall.h>
#include <unistd.h>

// for compat with older perf headers
#define uprobe_path config1

int main(int argc, char *argv[])
{
        // Find out what type id we need for uprobes
        int perf_type_pmu_uprobe;
        {
                FILE *fp = 
fopen("/sys/bus/event_source/devices/uprobe/type", "r");
                fscanf(fp, "%d", &perf_type_pmu_uprobe);
                fclose(fp);
        }

        const char *filename = "./bus";

        int fd = open(filename, O_RDWR|O_CREAT, 0600);
        write(fd, "x", 1);

        void *addr = mmap(NULL, 4096,
                PROT_READ | PROT_WRITE | PROT_EXEC,
                MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);

        // Register a perf uprobe on "./bus"
        struct perf_event_attr attr = {};
        attr.type = perf_type_pmu_uprobe;
        attr.uprobe_path = (unsigned long) filename;
        syscall(__NR_perf_event_open, &attr, 0, 0, -1, 0);

        void *addr2 = mmap(NULL, 2 * 4096,
                PROT_NONE,
                MAP_PRIVATE, fd, 0);
        void *addr3 = mremap((void *) addr2, 4096, 2 * 4096, 
MREMAP_MAYMOVE);
        mremap(addr3, 4096, 4096, MREMAP_MAYMOVE | MREMAP_FIXED, (void 
*) addr2);

        return 0;
}

this instantly reproduces this output on current mainline for me:

BUG: Bad rss-counter state mm:(____ptrval____) type:MM_ANONPAGES val:1

AFAICT the worst thing about this bug is that it shows up on anything
that parses logs for "BUG"; it doesn't seem to have any ill effects
other than messing up the rss counters. Although maybe it points to some
underlying problem in uprobes/mm interaction.

If I enable the "rss_stat" tracepoint and set ftrace_dump_on_oops=1, I
see a trace roughly like this:

perf_event_open()

mmap(2 * 4096):
 - uprobe_mmap()
    - install_breakpoint()
       - __replace_page()
          - rss_stat: mm_id=0 curr=1 member=1 size=53248B

mremap(4096 => 2 * 4096):
 - install_breakpoint()
    - __replace_page()
       - rss_stat: mm_id=0 curr=1 member=1 size=57344B
 - unmap_page_range()
    - rss_stat: mm_id=0 curr=1 member=1 size=53248B

mremap(4096 => 4096):
 - move_vma()
    - copy_vma()
       - vma_merge()
          - install_breakpoint()
             - __replace_page()
                - rss_stat: mm_id=0 curr=1 member=1 size=57344B
 - do_munmap()
    - install_breakpoint():
       - __replace_page()
          - rss_stat: mm_id=0 curr=1 member=1 size=61440B
    - unmap_page_range():
       - rss_stat: mm_id=0 curr=1 member=1 size=57344B

exit()
 - exit_mmap()
    - unmap_page_range():
       - rss_stat: mm_id=0 curr=0 member=1 size=45056B
    - unmap_page_range():
       - rss_stat: mm_id=0 curr=0 member=1 size=32768B
    - unmap_page_range():
       - rss_stat: mm_id=0 curr=0 member=1 size=20480B
    - unmap_page_range():
       - rss_stat: mm_id=0 curr=0 member=1 size=16384B
    - unmap_page_range():
       - rss_stat: mm_id=0 curr=0 member=1 size=4096B

What strikes me here is that at the end of the first mremap(), we have
size 53248B (13 pages), but at the end of the second mremap(), we have
size 57344B (14 pages), even though the second mremap() is only moving 1
page. So the second mremap() is bumping it up twice, but then only
bumping down once.


Vegard