Re: KASAN: use-after-free Read in blkdev_get

Re: KASAN: use-after-free Read in blkdev_get

[syzbot]

syzbot has bisected this bug to:

commit 77ef8f5177599efd0cedeb52c1950c1bd73fa5e3
Author: Chris Metcalf <cmetcalf@ezchip.com>
Date:   Mon Jan 25 20:05:34 2016 +0000

     tile kgdb: fix bug in copy to gdb regs, and optimize memset

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=1131bc0ee00000
start commit:   f5b7769e Revert "debugfs: inode: debugfs_create_dir uses m..
git tree:       upstream
final crash:    https://syzkaller.appspot.com/x/report.txt?x=1331bc0ee00000
console output: https://syzkaller.appspot.com/x/log.txt?x=1531bc0ee00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=709f8187af941e84
dashboard link: https://syzkaller.appspot.com/bug?extid=eaeb616d85c9a0afec7d
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=177f898f800000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=147eb85f800000

Reported-by: syzbot+eaeb616d85c9a0afec7d@syzkaller.appspotmail.com
Fixes: 77ef8f517759 ("tile kgdb: fix bug in copy to gdb regs, and optimize  
memset")

For information about bisection process see: https://goo.gl/tpsmEJ#bisection

Re: KASAN: use-after-free Read in blkdev_get

[Al Viro]

On Sun, Nov 24, 2019 at 11:07:00AM -0800, syzbot wrote:
> syzbot has bisected this bug to:
> 
> commit 77ef8f5177599efd0cedeb52c1950c1bd73fa5e3
> Author: Chris Metcalf <cmetcalf@ezchip.com>
> Date:   Mon Jan 25 20:05:34 2016 +0000
> 
>     tile kgdb: fix bug in copy to gdb regs, and optimize memset
> 
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=1131bc0ee00000
> start commit:   f5b7769e Revert "debugfs: inode: debugfs_create_dir uses m..
> git tree:       upstream
> final crash:    https://syzkaller.appspot.com/x/report.txt?x=1331bc0ee00000
> console output: https://syzkaller.appspot.com/x/log.txt?x=1531bc0ee00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=709f8187af941e84
> dashboard link: https://syzkaller.appspot.com/bug?extid=eaeb616d85c9a0afec7d
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=177f898f800000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=147eb85f800000
> 
> Reported-by: syzbot+eaeb616d85c9a0afec7d@syzkaller.appspotmail.com
> Fixes: 77ef8f517759 ("tile kgdb: fix bug in copy to gdb regs, and optimize
> memset")
> 
> For information about bisection process see: https://goo.gl/tpsmEJ#bisection

Seriously?  How can the commit in question (limited to arch/tile/kernel/kgdb.c)
possibly affect a bug that manages to produce a crash report with
RSP: 0018:ffffffff82e03eb8  EFLAGS: 00000282                                    
RAX: 0000000000000000 RBX: ffffffff82e00000 RCX: 0000000000000000               
RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffffff81088779               
RBP: ffffffff82e03eb8 R08: 0000000000000000 R09: 0000000000000001               
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000               
R13: 0000000000000000 R14: 0000000000000000 R15: ffffffff82e00000               
FS:  0000000000000000(0000) GS:ffff88021fc00000(0000) knlGS:0000000000000000    
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033                               
CR2: 000000c420447ff8 CR3: 0000000213184000 CR4: 00000000001406f0               
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000               
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400               
in it?  Unless something very odd has happened to tile, this crash has
been observed on 64bit x86; the names of registers alone are enough
to be certain of that.

And the binaries produced by an x86 build should not be affected by any
changes in arch/tile; not unless something is very wrong with the build
system.  It's not even that this commit has fixed an earlier bug that
used to mask the one manifested here - it really should have had zero
impact on x86 builds, period.

So I'm sorry, but I'm calling bullshit.  Something's quite wrong with
the bot - either its build system or the bisection process.

Re: KASAN: use-after-free Read in blkdev_get

[Duncan Roe]

On Sun, Nov 24, 2019 at 07:30:35PM +0000, Al Viro wrote:
> On Sun, Nov 24, 2019 at 11:07:00AM -0800, syzbot wrote:
> > syzbot has bisected this bug to:
> >
> > commit 77ef8f5177599efd0cedeb52c1950c1bd73fa5e3
> > Author: Chris Metcalf <cmetcalf@ezchip.com>
> > Date:   Mon Jan 25 20:05:34 2016 +0000
> >
> >     tile kgdb: fix bug in copy to gdb regs, and optimize memset
> >
> > bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=1131bc0ee00000
> > start commit:   f5b7769e Revert "debugfs: inode: debugfs_create_dir uses m..
> > git tree:       upstream
> > final crash:    https://syzkaller.appspot.com/x/report.txt?x=1331bc0ee00000
> > console output: https://syzkaller.appspot.com/x/log.txt?x=1531bc0ee00000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=709f8187af941e84
> > dashboard link: https://syzkaller.appspot.com/bug?extid=eaeb616d85c9a0afec7d
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=177f898f800000
> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=147eb85f800000
> >
> > Reported-by: syzbot+eaeb616d85c9a0afec7d@syzkaller.appspotmail.com
> > Fixes: 77ef8f517759 ("tile kgdb: fix bug in copy to gdb regs, and optimize
> > memset")
> >
> > For information about bisection process see: https://goo.gl/tpsmEJ#bisection
>
> Seriously?  How can the commit in question (limited to arch/tile/kernel/kgdb.c)
> possibly affect a bug that manages to produce a crash report with
> RSP: 0018:ffffffff82e03eb8  EFLAGS: 00000282
> RAX: 0000000000000000 RBX: ffffffff82e00000 RCX: 0000000000000000
> RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffffff81088779
> RBP: ffffffff82e03eb8 R08: 0000000000000000 R09: 0000000000000001
> R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
> R13: 0000000000000000 R14: 0000000000000000 R15: ffffffff82e00000
> FS:  0000000000000000(0000) GS:ffff88021fc00000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 000000c420447ff8 CR3: 0000000213184000 CR4: 00000000001406f0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> in it?  Unless something very odd has happened to tile, this crash has
> been observed on 64bit x86; the names of registers alone are enough
> to be certain of that.
>
> And the binaries produced by an x86 build should not be affected by any
> changes in arch/tile; not unless something is very wrong with the build
> system.  It's not even that this commit has fixed an earlier bug that
> used to mask the one manifested here - it really should have had zero
> impact on x86 builds, period.
>
> So I'm sorry, but I'm calling bullshit.  Something's quite wrong with
> the bot - either its build system or the bisection process.

The acid test would be: does reverting that commit make the problem go away?

See, for example, https://bugzilla.kernel.org/show_bug.cgi?id=203935

Cheers ... Duncan.

Re: KASAN: use-after-free Read in blkdev_get

[Dmitry Vyukov]

On Sat, Nov 30, 2019 at 12:06 PM Duncan Roe <duncan_roe@optusnet.com.au> wrote:
> > > syzbot has bisected this bug to:
> > >
> > > commit 77ef8f5177599efd0cedeb52c1950c1bd73fa5e3
> > > Author: Chris Metcalf <cmetcalf@ezchip.com>
> > > Date:   Mon Jan 25 20:05:34 2016 +0000
> > >
> > >     tile kgdb: fix bug in copy to gdb regs, and optimize memset
> > >
> > > bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=1131bc0ee00000
> > > start commit:   f5b7769e Revert "debugfs: inode: debugfs_create_dir uses m..
> > > git tree:       upstream
> > > final crash:    https://syzkaller.appspot.com/x/report.txt?x=1331bc0ee00000
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=1531bc0ee00000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=709f8187af941e84
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=eaeb616d85c9a0afec7d
> > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=177f898f800000
> > > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=147eb85f800000
> > >
> > > Reported-by: syzbot+eaeb616d85c9a0afec7d@syzkaller.appspotmail.com
> > > Fixes: 77ef8f517759 ("tile kgdb: fix bug in copy to gdb regs, and optimize
> > > memset")
> > >
> > > For information about bisection process see: https://goo.gl/tpsmEJ#bisection
> >
> > Seriously?  How can the commit in question (limited to arch/tile/kernel/kgdb.c)
> > possibly affect a bug that manages to produce a crash report with
> > RSP: 0018:ffffffff82e03eb8  EFLAGS: 00000282
> > RAX: 0000000000000000 RBX: ffffffff82e00000 RCX: 0000000000000000
> > RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffffff81088779
> > RBP: ffffffff82e03eb8 R08: 0000000000000000 R09: 0000000000000001
> > R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
> > R13: 0000000000000000 R14: 0000000000000000 R15: ffffffff82e00000
> > FS:  0000000000000000(0000) GS:ffff88021fc00000(0000) knlGS:0000000000000000
> > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: 000000c420447ff8 CR3: 0000000213184000 CR4: 00000000001406f0
> > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > in it?  Unless something very odd has happened to tile, this crash has
> > been observed on 64bit x86; the names of registers alone are enough
> > to be certain of that.
> >
> > And the binaries produced by an x86 build should not be affected by any
> > changes in arch/tile; not unless something is very wrong with the build
> > system.  It's not even that this commit has fixed an earlier bug that
> > used to mask the one manifested here - it really should have had zero
> > impact on x86 builds, period.
> >
> > So I'm sorry, but I'm calling bullshit.  Something's quite wrong with
> > the bot - either its build system or the bisection process.
>
> The acid test would be: does reverting that commit make the problem go away?
>
> See, for example, https://bugzilla.kernel.org/show_bug.cgi?id=203935
>
> Cheers ... Duncan.

This is done as part of any bisection by definition, right? The test
was done on the previous commit (effectively this one reverted) and no
crash was observed. Otherwise bisection would have been pointed to a
different commit.

Re: KASAN: use-after-free Read in blkdev_get

[Duncan Roe]

On Sat, Nov 30, 2019 at 04:53:12PM +0100, Dmitry Vyukov wrote:
> On Sat, Nov 30, 2019 at 12:06 PM Duncan Roe <duncan_roe@optusnet.com.au> wrote:
> > > > syzbot has bisected this bug to:
> > > >
> > > > commit 77ef8f5177599efd0cedeb52c1950c1bd73fa5e3
> > > > Author: Chris Metcalf <cmetcalf@ezchip.com>
> > > > Date:   Mon Jan 25 20:05:34 2016 +0000
> > > >
> > > >     tile kgdb: fix bug in copy to gdb regs, and optimize memset
> > > >
> > > > bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=1131bc0ee00000
> > > > start commit:   f5b7769e Revert "debugfs: inode: debugfs_create_dir uses m..
> > > > git tree:       upstream
> > > > final crash:    https://syzkaller.appspot.com/x/report.txt?x=1331bc0ee00000
> > > > console output: https://syzkaller.appspot.com/x/log.txt?x=1531bc0ee00000
> > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=709f8187af941e84
> > > > dashboard link: https://syzkaller.appspot.com/bug?extid=eaeb616d85c9a0afec7d
> > > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=177f898f800000
> > > > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=147eb85f800000
> > > >
> > > > Reported-by: syzbot+eaeb616d85c9a0afec7d@syzkaller.appspotmail.com
> > > > Fixes: 77ef8f517759 ("tile kgdb: fix bug in copy to gdb regs, and optimize
> > > > memset")
> > > >
> > > > For information about bisection process see: https://goo.gl/tpsmEJ#bisection
> > >
> > > Seriously?  How can the commit in question (limited to arch/tile/kernel/kgdb.c)
> > > possibly affect a bug that manages to produce a crash report with
> > > RSP: 0018:ffffffff82e03eb8  EFLAGS: 00000282
> > > RAX: 0000000000000000 RBX: ffffffff82e00000 RCX: 0000000000000000
> > > RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffffff81088779
> > > RBP: ffffffff82e03eb8 R08: 0000000000000000 R09: 0000000000000001
> > > R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
> > > R13: 0000000000000000 R14: 0000000000000000 R15: ffffffff82e00000
> > > FS:  0000000000000000(0000) GS:ffff88021fc00000(0000) knlGS:0000000000000000
> > > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > CR2: 000000c420447ff8 CR3: 0000000213184000 CR4: 00000000001406f0
> > > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > > in it?  Unless something very odd has happened to tile, this crash has
> > > been observed on 64bit x86; the names of registers alone are enough
> > > to be certain of that.
> > >
> > > And the binaries produced by an x86 build should not be affected by any
> > > changes in arch/tile; not unless something is very wrong with the build
> > > system.  It's not even that this commit has fixed an earlier bug that
> > > used to mask the one manifested here - it really should have had zero
> > > impact on x86 builds, period.
> > >
> > > So I'm sorry, but I'm calling bullshit.  Something's quite wrong with
> > > the bot - either its build system or the bisection process.
> >
> > The acid test would be: does reverting that commit make the problem go away?
> >
> > See, for example, https://bugzilla.kernel.org/show_bug.cgi?id=203935
> >
> > Cheers ... Duncan.
>
> This is done as part of any bisection by definition, right? The test
> was done on the previous commit (effectively this one reverted) and no
> crash was observed. Otherwise bisection would have been pointed to a
> different commit.
>
Agree that's what bisecting does. What I had in mind was to make a patch to
remove the identified commit, and apply that to the most recent revision
possible. Then see if that makes the problem go away.

However when I look at my clone of
git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git and compare
77ef8f5177 with the previous commit a7d1357664, "git diff --stat" shows

> 745 files changed, 8192 insertions(+), 15408 deletions(-)

The last few lines from a grep of "arch" are:

> arch/s390/oprofile/backtrace.c                     |    8 +-
> arch/tile/kernel/kgdb.c                            |    8 +-
> arch/x86/Kconfig                                   |    5 +-
> arch/x86/include/asm/livepatch.h                   |    2 +-
> arch/x86/include/asm/processor.h                   |    2 +-
> arch/x86/kernel/cpu/perf_event_amd_uncore.c        |    2 +
> arch/x86/lib/copy_user_64.S                        |  142 ++-
> arch/x86/mm/fault.c                                |   15 +-
> arch/x86/mm/gup.c                                  |    2 +-
> arch/x86/mm/numa.c                                 |    2 +-

Enough said?

Cheers ... Duncan.

Re: KASAN: use-after-free Read in blkdev_get

[Dmitry Vyukov]

On Sun, Dec 1, 2019 at 1:04 AM Duncan Roe <duncan_roe@optusnet.com.au> wrote:
>
> On Sat, Nov 30, 2019 at 04:53:12PM +0100, Dmitry Vyukov wrote:
> > On Sat, Nov 30, 2019 at 12:06 PM Duncan Roe <duncan_roe@optusnet.com.au> wrote:
> > > > > syzbot has bisected this bug to:
> > > > >
> > > > > commit 77ef8f5177599efd0cedeb52c1950c1bd73fa5e3
> > > > > Author: Chris Metcalf <cmetcalf@ezchip.com>
> > > > > Date:   Mon Jan 25 20:05:34 2016 +0000
> > > > >
> > > > >     tile kgdb: fix bug in copy to gdb regs, and optimize memset
> > > > >
> > > > > bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=1131bc0ee00000
> > > > > start commit:   f5b7769e Revert "debugfs: inode: debugfs_create_dir uses m..
> > > > > git tree:       upstream
> > > > > final crash:    https://syzkaller.appspot.com/x/report.txt?x=1331bc0ee00000
> > > > > console output: https://syzkaller.appspot.com/x/log.txt?x=1531bc0ee00000
> > > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=709f8187af941e84
> > > > > dashboard link: https://syzkaller.appspot.com/bug?extid=eaeb616d85c9a0afec7d
> > > > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=177f898f800000
> > > > > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=147eb85f800000
> > > > >
> > > > > Reported-by: syzbot+eaeb616d85c9a0afec7d@syzkaller.appspotmail.com
> > > > > Fixes: 77ef8f517759 ("tile kgdb: fix bug in copy to gdb regs, and optimize
> > > > > memset")
> > > > >
> > > > > For information about bisection process see: https://goo.gl/tpsmEJ#bisection
> > > >
> > > > Seriously?  How can the commit in question (limited to arch/tile/kernel/kgdb.c)
> > > > possibly affect a bug that manages to produce a crash report with
> > > > RSP: 0018:ffffffff82e03eb8  EFLAGS: 00000282
> > > > RAX: 0000000000000000 RBX: ffffffff82e00000 RCX: 0000000000000000
> > > > RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffffff81088779
> > > > RBP: ffffffff82e03eb8 R08: 0000000000000000 R09: 0000000000000001
> > > > R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
> > > > R13: 0000000000000000 R14: 0000000000000000 R15: ffffffff82e00000
> > > > FS:  0000000000000000(0000) GS:ffff88021fc00000(0000) knlGS:0000000000000000
> > > > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > > CR2: 000000c420447ff8 CR3: 0000000213184000 CR4: 00000000001406f0
> > > > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > > > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > > > in it?  Unless something very odd has happened to tile, this crash has
> > > > been observed on 64bit x86; the names of registers alone are enough
> > > > to be certain of that.
> > > >
> > > > And the binaries produced by an x86 build should not be affected by any
> > > > changes in arch/tile; not unless something is very wrong with the build
> > > > system.  It's not even that this commit has fixed an earlier bug that
> > > > used to mask the one manifested here - it really should have had zero
> > > > impact on x86 builds, period.
> > > >
> > > > So I'm sorry, but I'm calling bullshit.  Something's quite wrong with
> > > > the bot - either its build system or the bisection process.
> > >
> > > The acid test would be: does reverting that commit make the problem go away?
> > >
> > > See, for example, https://bugzilla.kernel.org/show_bug.cgi?id=203935
> > >
> > > Cheers ... Duncan.
> >
> > This is done as part of any bisection by definition, right? The test
> > was done on the previous commit (effectively this one reverted) and no
> > crash was observed. Otherwise bisection would have been pointed to a
> > different commit.
> >
> Agree that's what bisecting does. What I had in mind was to make a patch to
> remove the identified commit, and apply that to the most recent revision
> possible. Then see if that makes the problem go away.

I wonder in what percent of cases:
1. It gives signal different from reverting the commit in place.
2. The revert can be cleanly applied to head.
3. The revert does not introduce other bugs.

For this to be worth doing, all these 3 should be reasonably high. I
can imagine 3 may be high (?), but I am not sure about 1 and 2.

Re: KASAN: use-after-free Read in blkdev_get

[Duncan Roe]

On Mon, Dec 02, 2019 at 07:47:11AM +0100, Dmitry Vyukov wrote:
> On Sun, Dec 1, 2019 at 1:04 AM Duncan Roe <duncan_roe@optusnet.com.au> wrote:
> >
> > On Sat, Nov 30, 2019 at 04:53:12PM +0100, Dmitry Vyukov wrote:
> > > On Sat, Nov 30, 2019 at 12:06 PM Duncan Roe <duncan_roe@optusnet.com.au> wrote:
> > > > > > syzbot has bisected this bug to:
> > > > > >
> > > > > > commit 77ef8f5177599efd0cedeb52c1950c1bd73fa5e3
> > > > > > Author: Chris Metcalf <cmetcalf@ezchip.com>
> > > > > > Date:   Mon Jan 25 20:05:34 2016 +0000
> > > > > >
> > > > > >     tile kgdb: fix bug in copy to gdb regs, and optimize memset
> > > > > >
> > > > > > bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=1131bc0ee00000
> > > > > > start commit:   f5b7769e Revert "debugfs: inode: debugfs_create_dir uses m..
> > > > > > git tree:       upstream
> > > > > > final crash:    https://syzkaller.appspot.com/x/report.txt?x=1331bc0ee00000
> > > > > > console output: https://syzkaller.appspot.com/x/log.txt?x=1531bc0ee00000
> > > > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=709f8187af941e84
> > > > > > dashboard link: https://syzkaller.appspot.com/bug?extid=eaeb616d85c9a0afec7d
> > > > > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=177f898f800000
> > > > > > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=147eb85f800000
> > > > > >
> > > > > > Reported-by: syzbot+eaeb616d85c9a0afec7d@syzkaller.appspotmail.com
> > > > > > Fixes: 77ef8f517759 ("tile kgdb: fix bug in copy to gdb regs, and optimize
> > > > > > memset")
> > > > > >
> > > > > > For information about bisection process see: https://goo.gl/tpsmEJ#bisection
> > > > >
> > > > > Seriously?  How can the commit in question (limited to arch/tile/kernel/kgdb.c)
> > > > > possibly affect a bug that manages to produce a crash report with
> > > > > RSP: 0018:ffffffff82e03eb8  EFLAGS: 00000282
> > > > > RAX: 0000000000000000 RBX: ffffffff82e00000 RCX: 0000000000000000
> > > > > RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffffff81088779
> > > > > RBP: ffffffff82e03eb8 R08: 0000000000000000 R09: 0000000000000001
> > > > > R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
> > > > > R13: 0000000000000000 R14: 0000000000000000 R15: ffffffff82e00000
> > > > > FS:  0000000000000000(0000) GS:ffff88021fc00000(0000) knlGS:0000000000000000
> > > > > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > > > CR2: 000000c420447ff8 CR3: 0000000213184000 CR4: 00000000001406f0
> > > > > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > > > > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > > > > in it?  Unless something very odd has happened to tile, this crash has
> > > > > been observed on 64bit x86; the names of registers alone are enough
> > > > > to be certain of that.
> > > > >
> > > > > And the binaries produced by an x86 build should not be affected by any
> > > > > changes in arch/tile; not unless something is very wrong with the build
> > > > > system.  It's not even that this commit has fixed an earlier bug that
> > > > > used to mask the one manifested here - it really should have had zero
> > > > > impact on x86 builds, period.
> > > > >
> > > > > So I'm sorry, but I'm calling bullshit.  Something's quite wrong with
> > > > > the bot - either its build system or the bisection process.
> > > >
> > > > The acid test would be: does reverting that commit make the problem go away?
> > > >
> > > > See, for example, https://bugzilla.kernel.org/show_bug.cgi?id=203935
> > > >
> > > > Cheers ... Duncan.
> > >
> > > This is done as part of any bisection by definition, right? The test
> > > was done on the previous commit (effectively this one reverted) and no
> > > crash was observed. Otherwise bisection would have been pointed to a
> > > different commit.
> > >
> > Agree that's what bisecting does. What I had in mind was to make a patch to
> > remove the identified commit, and apply that to the most recent revision
> > possible. Then see if that makes the problem go away.
>
> I wonder in what percent of cases:
> 1. It gives signal different from reverting the commit in place.
> 2. The revert can be cleanly applied to head.
> 3. The revert does not introduce other bugs.
>
> For this to be worth doing, all these 3 should be reasonably high. I
> can imagine 3 may be high (?), but I am not sure about 1 and 2.

The whole arch/tile directory no longer exists, so the patch cannot be applied.
If I had realised that earlier, I would not have posted at all.
Sorry for the noise.