RE: [PATCH] kcov: add KCOV_PC_RANGE to limit pc range

From: JianGen Jiao (QUIC)
Date: Sun Nov 21 2021 - 22:44:18 EST

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https://stackoverflow.com/questions/43003805/can-ebpf-modify-the-return-value-or-parameters-of-a-syscall
eBPF seems read-only, I think it won't overcome the area overflow if cannot modify the area inside kernel?

-----Original Message-----
From: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Sent: Friday, November 19, 2021 10:14 PM
To: Andrey Konovalov <andreyknvl@xxxxxxxxx>
Cc: JianGen Jiao (QUIC) <quic_jiangenj@xxxxxxxxxxx>; syzkaller <syzkaller@xxxxxxxxxxxxxxxx>; kasan-dev@xxxxxxxxxxxxxxxx; LKML <linux-kernel@xxxxxxxxxxxxxxx>; Alexander Lochmann <info@xxxxxxxxxxxxxxxxxxxxx>; Likai Ding (QUIC) <quic_likaid@xxxxxxxxxxx>; Kaipeng Zeng <kaipeng94@xxxxxxxxx>; Hangbin Liu <liuhangbin@xxxxxxxxx>
Subject: Re: [PATCH] kcov: add KCOV_PC_RANGE to limit pc range

On Fri, 19 Nov 2021 at 15:09, Andrey Konovalov <andreyknvl@xxxxxxxxx> wrote:
> > > +Kaipeng, Hangbin who contributed the coverage filter to syzkaller.
> > > This is a discussion about adding a similar filter to the kernel.
> > > You can see whole discussion here:
> > > https://groups.google.com/g/kasan-dev/c/TQmYUdUC08Y
> >
> > Joey, what do you think in general about passing a filter bitmap to the kernel?
> >
> > Since the bitmap is large, it can make sense to reuse it across
> > different KCOV instances.
> > I am thinking about something along the following lines:
> >
> > kcov_fd = open("/debugfs/kcov");
> > filter_fd = ioctl(kcov_fd, KCOV_CREATE_FILTER, &{... some args
> > specifying start/end ...}); filter = mmap(..., filter_fd); ... write
> > to the filter ...
> >
> > ...
> > kcov_fd2 = open("/debugfs/kcov");
> > ioctl(kcov_fd2, KCOV_ATTACH_FILTER, filter_fd); ioctl(kcov_fd2,
> > KCOV_ENABLE);
> >
> >
> > This would allow us to create 2 filters:
> > 1. One the interesting subsystems
> > 2. Second only for yet uncovered PCs in the interesting subsystems
> > (updated as we discover more coverage)
> >
> > During fuzzing we attach the second filter to KCOV.
> > But when we want to obtain full program coverage, we attach the first one.
> >
> > The filters (bitmaps) are reused across all threads in all executor
> > processes (so that we have only 2 filters globally per VM).
> >
> > KCOV_CREATE_FILTER could also accept how many bytes each bit
> > represents (that scaling factor, as hardcoding 4, 8, 16 may be bad
> > for a stable kernel interface).
> >
> > But I am still not sure how to support both the main kernel and
> > modules. We could allow setting up multiple filters for different PC
> > ranges. Or may be just 2 (one for kernel and one for modules range).
> > Or maybe 1 bitmap can cover both kernel and modules?
> >
> > Thoughts?
>
> Throwing in a thought without a concrete design suggestion: how about
> en eBPF-based filter? The flexibility would allow covering as many PC
> ranges as one wants. And, perhaps, do other things.

This is definitely interesting and flexible. eBPF have different types of maps nowadays and these can be accessed from user-space as well.
Alternatively could we attach just an eBPF map as a filter?
But we would need to measure overhead, this will be executed for every basic block of code.

> > > On Fri, 19 Nov 2021 at 12:21, JianGen Jiao (QUIC)
> > > <quic_jiangenj@xxxxxxxxxxx> wrote:
> > > >
> > > > Yes, on x86_64, module address space is after kernel. But like below on arm64, it's different.
> > > >
> > > > # grep stext /proc/kallsyms
> > > > ffffffc010010000 T _stext
> > > > # cat /proc/modules |sort -k 6 | tail -2
> > > > Some_module_1 552960 0 - Live 0xffffffc00ca05000 (O)
> > > > Some_module_1 360448 0 - Live 0xffffffc00cb8f000 (O) # cat
> > > > /proc/modules |sort -k 6 | head -2
> > > > Some_module_3 16384 1 - Live 0xffffffc009430000
> > > >
> > > > -----Original Message-----
> > > > From: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> > > > Sent: Friday, November 19, 2021 6:38 PM
> > > > To: JianGen Jiao (QUIC) <quic_jiangenj@xxxxxxxxxxx>
> > > > Cc: andreyknvl@xxxxxxxxx; kasan-dev@xxxxxxxxxxxxxxxx; LKML
> > > > <linux-kernel@xxxxxxxxxxxxxxx>; Alexander Lochmann
> > > > <info@xxxxxxxxxxxxxxxxxxxxx>; Likai Ding (QUIC)
> > > > <quic_likaid@xxxxxxxxxxx>
> > > > Subject: Re: [PATCH] kcov: add KCOV_PC_RANGE to limit pc range
> > > >
> > > > WARNING: This email originated from outside of Qualcomm. Please be wary of any links or attachments, and do not enable macros.
> > > >
> > > > On Fri, 19 Nov 2021 at 04:17, JianGen Jiao (QUIC) <quic_jiangenj@xxxxxxxxxxx> wrote:
> > > > >
> > > > > Hi Dmitry,
> > > > > I'm using the start, end pc from cover filter, which currently is the fast way compared to the big bitmap passing from syzkaller solution, as I only set the cover filter to dirs/files I care about.
> > > >
> > > > I see.
> > > > But if we are unlucky and our functions of interest are at the very low and high addresses, start/end will cover almost all kernel code...
> > > >
> > > > > I checked
> > > > > https://groups.google.com/g/kasan-dev/c/oVz3ZSWaK1Q/m/9ASztdzC
> > > > > AAAJ, The bitmap seems not the same as syzkaller one, which
> > > > > one will be used finally?
> > > >
> > > > I don't know yet. We need to decide.
> > > > In syzkaller we are more flexible and can change code faster, while kernel interfaces are stable and need to be kept forever. So I think we need to concentrate more on the good kernel interface and then support it in syzkaller.
> > > >
> > > > > ``` Alexander's one
> > > > > + pos = (ip - canonicalize_ip((unsigned long)&_stext)) / 4;
> > > > > + idx = pos % BITS_PER_LONG; pos /= BITS_PER_LONG; if
> > > > > + (likely(pos <
> > > > > + t->kcov_size)) WRITE_ONCE(area[pos], READ_ONCE(area[pos]) |
> > > > > + t->1L <<
> > > > > + idx);
> > > > > ```
> > > > > Pc offset is divided by 4 and start is _stext. But for some arch, pc is less than _stext.
> > > >
> > > > You mean that modules can have PC < _stext?
> > > >
> > > > > ``` https://github.com/google/syzkaller/blob/master/syz-manager/covfilter.go#L139-L154
> > > > > data := make([]byte, 8+((size>>4)/8+1))
> > > > > order := binary.ByteOrder(binary.BigEndian)
> > > > > if target.LittleEndian {
> > > > > order = binary.LittleEndian
> > > > > }
> > > > > order.PutUint32(data, start)
> > > > > order.PutUint32(data[4:], size)
> > > > >
> > > > > bitmap := data[8:]
> > > > > for pc := range pcs {
> > > > > // The lowest 4-bit is dropped.
> > > > > pc = uint32(backend.NextInstructionPC(target, uint64(pc)))
> > > > > pc = (pc - start) >> 4
> > > > > bitmap[pc/8] |= (1 << (pc % 8))
> > > > > }
> > > > > return data
> > > > > ```
> > > > > Pc offset is divided by 16 and start is cover filter start pc.
> > > > >
> > > > > I think divided by 8 is more reasonable? Because there is at least one instruction before each __sanitizer_cov_trace_pc call.
> > > > > 0000000000000160 R_AARCH64_CALL26 __sanitizer_cov_trace_pc
> > > > > 0000000000000168 R_AARCH64_CALL26 __sanitizer_cov_trace_pc
> > > > >
> > > > > I think we still need my patch because we still need a way to keep the trace_pc call and post-filter in syzkaller doesn't solve trace_pc dropping, right?
> > > >
> > > > Yes, the in-kernel filter solves the problem of trace capacity/overflows.
> > > >
> > > >
> > > > > But for sure I can use the bitmap from syzkaller.
> > > > >
> > > > > THX
> > > > > Joey
> > > > > -----Original Message-----
> > > > > From: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> > > > > Sent: Thursday, November 18, 2021 10:00 PM
> > > > > To: JianGen Jiao (QUIC) <quic_jiangenj@xxxxxxxxxxx>
> > > > > Cc: andreyknvl@xxxxxxxxx; kasan-dev@xxxxxxxxxxxxxxxx; LKML
> > > > > <linux-kernel@xxxxxxxxxxxxxxx>; Alexander Lochmann
> > > > > <info@xxxxxxxxxxxxxxxxxxxxx>
> > > > > Subject: Re: [PATCH] kcov: add KCOV_PC_RANGE to limit pc range
> > > > >
> > > > > WARNING: This email originated from outside of Qualcomm. Please be wary of any links or attachments, and do not enable macros.
> > > > >
> > > > > ,On Wed, 17 Nov 2021 at 07:24, Joey Jiao <quic_jiangenj@xxxxxxxxxxx> wrote:
> > > > > >
> > > > > > Sometimes we only interested in the pcs within some range,
> > > > > > while there are cases these pcs are dropped by kernel due to
> > > > > > `pos >=
> > > > > > t->kcov_size`, and by increasing the map area size doesn't help.
> > > > > >
> > > > > > To avoid disabling KCOV for these not intereseted pcs during
> > > > > > build time, adding this new KCOV_PC_RANGE cmd.
> > > > >
> > > > > Hi Joey,
> > > > >
> > > > > How do you use this? I am concerned that a single range of PCs is too restrictive. I can only see how this can work for single module (continuous in memory) or a single function. But for anything else (something in the main kernel, or several modules), it won't work as PCs are not continuous.
> > > > >
> > > > > Maybe we should use a compressed bitmap of interesting PCs? It allows to support all cases and we already have it in syz-executor, then syz-executor could simply pass the bitmap to the kernel rather than post-filter.
> > > > > It's also overlaps with the KCOV_MODE_UNIQUE mode that +Alexander proposed here:
> > > > > https://groups.google.com/g/kasan-dev/c/oVz3ZSWaK1Q/m/9ASztdzC
> > > > > AAAJ It would be reasonable if kernel uses the same bitmap
> > > > > format for these
> > > > > 2 features.
> > > > >
> > > > >
> > > > >
> > > > > > An example usage is to use together syzkaller's cov filter.
> > > > > >
> > > > > > Change-Id: I954f6efe1bca604f5ce31f8f2b6f689e34a2981d
> > > > > > Signed-off-by: Joey Jiao <quic_jiangenj@xxxxxxxxxxx>
> > > > > > ---
> > > > > > Documentation/dev-tools/kcov.rst | 10 ++++++++++
> > > > > > include/uapi/linux/kcov.h | 7 +++++++
> > > > > > kernel/kcov.c | 18 ++++++++++++++++++
> > > > > > 3 files changed, 35 insertions(+)
> > > > > >
> > > > > > diff --git a/Documentation/dev-tools/kcov.rst
> > > > > > b/Documentation/dev-tools/kcov.rst
> > > > > > index d83c9ab..fbcd422 100644
> > > > > > --- a/Documentation/dev-tools/kcov.rst
> > > > > > +++ b/Documentation/dev-tools/kcov.rst
> > > > > > @@ -52,9 +52,15 @@ program using kcov:
> > > > > > #include <fcntl.h>
> > > > > > #include <linux/types.h>
> > > > > >
> > > > > > + struct kcov_pc_range {
> > > > > > + uint32 start;
> > > > > > + uint32 end;
> > > > > > + };
> > > > > > +
> > > > > > #define KCOV_INIT_TRACE _IOR('c', 1, unsigned long)
> > > > > > #define KCOV_ENABLE _IO('c', 100)
> > > > > > #define KCOV_DISABLE _IO('c', 101)
> > > > > > + #define KCOV_TRACE_RANGE _IOW('c', 103, struct kcov_pc_range)
> > > > > > #define COVER_SIZE (64<<10)
> > > > > >
> > > > > > #define KCOV_TRACE_PC 0 @@ -64,6 +70,8 @@ program
> > > > > > using kcov:
> > > > > > {
> > > > > > int fd;
> > > > > > unsigned long *cover, n, i;
> > > > > > + /* Change start and/or end to your interested pc range. */
> > > > > > + struct kcov_pc_range pc_range = {.start = 0, .end =
> > > > > > + (uint32)(~((uint32)0))};
> > > > > >
> > > > > > /* A single fd descriptor allows coverage collection on a single
> > > > > > * thread.
> > > > > > @@ -79,6 +87,8 @@ program using kcov:
> > > > > > PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
> > > > > > if ((void*)cover == MAP_FAILED)
> > > > > > perror("mmap"), exit(1);
> > > > > > + if (ioctl(fd, KCOV_PC_RANGE, pc_range))
> > > > > > + dprintf(2, "ignore KCOV_PC_RANGE error.\n");
> > > > > > /* Enable coverage collection on the current thread. */
> > > > > > if (ioctl(fd, KCOV_ENABLE, KCOV_TRACE_PC))
> > > > > > perror("ioctl"), exit(1); diff --git
> > > > > > a/include/uapi/linux/kcov.h b/include/uapi/linux/kcov.h
> > > > > > index 1d0350e..353ff0a 100644
> > > > > > --- a/include/uapi/linux/kcov.h
> > > > > > +++ b/include/uapi/linux/kcov.h
> > > > > > @@ -16,12 +16,19 @@ struct kcov_remote_arg {
> > > > > > __aligned_u64 handles[0];
> > > > > > };
> > > > > >
> > > > > > +#define PC_RANGE_MASK ((__u32)(~((u32) 0))) struct kcov_pc_range {
> > > > > > + __u32 start; /* start pc & 0xFFFFFFFF */
> > > > > > + __u32 end; /* end pc & 0xFFFFFFFF */
> > > > > > +};
> > > > > > +
> > > > > > #define KCOV_REMOTE_MAX_HANDLES 0x100
> > > > > >
> > > > > > #define KCOV_INIT_TRACE _IOR('c', 1, unsigned long)
> > > > > > #define KCOV_ENABLE _IO('c', 100)
> > > > > > #define KCOV_DISABLE _IO('c', 101)
> > > > > > #define KCOV_REMOTE_ENABLE _IOW('c', 102, struct kcov_remote_arg)
> > > > > > +#define KCOV_PC_RANGE _IOW('c', 103, struct kcov_pc_range)
> > > > > >
> > > > > > enum {
> > > > > > /*
> > > > > > diff --git a/kernel/kcov.c b/kernel/kcov.c index
> > > > > > 36ca640..59550450
> > > > > > 100644
> > > > > > --- a/kernel/kcov.c
> > > > > > +++ b/kernel/kcov.c
> > > > > > @@ -36,6 +36,7 @@
> > > > > > * - initial state after open()
> > > > > > * - then there must be a single ioctl(KCOV_INIT_TRACE) call
> > > > > > * - then, mmap() call (several calls are allowed but not
> > > > > > useful)
> > > > > > + * - then, optional to set trace pc range
> > > > > > * - then, ioctl(KCOV_ENABLE, arg), where arg is
> > > > > > * KCOV_TRACE_PC - to trace only the PCs
> > > > > > * or
> > > > > > @@ -69,6 +70,8 @@ struct kcov {
> > > > > > * kcov_remote_stop(), see the comment there.
> > > > > > */
> > > > > > int sequence;
> > > > > > + /* u32 Trace PC range from start to end. */
> > > > > > + struct kcov_pc_range pc_range;
> > > > > > };
> > > > > >
> > > > > > struct kcov_remote_area {
> > > > > > @@ -192,6 +195,7 @@ static notrace unsigned long
> > > > > > canonicalize_ip(unsigned long ip) void notrace
> > > > > > __sanitizer_cov_trace_pc(void) {
> > > > > > struct task_struct *t;
> > > > > > + struct kcov_pc_range pc_range;
> > > > > > unsigned long *area;
> > > > > > unsigned long ip = canonicalize_ip(_RET_IP_);
> > > > > > unsigned long pos;
> > > > > > @@ -199,6 +203,11 @@ void notrace __sanitizer_cov_trace_pc(void)
> > > > > > t = current;
> > > > > > if (!check_kcov_mode(KCOV_MODE_TRACE_PC, t))
> > > > > > return;
> > > > > > + pc_range = t->kcov->pc_range;
> > > > > > + if (pc_range.start < pc_range.end &&
> > > > > > + ((ip & PC_RANGE_MASK) < pc_range.start ||
> > > > > > + (ip & PC_RANGE_MASK) > pc_range.end))
> > > > > > + return;
> > > > > >
> > > > > > area = t->kcov_area;
> > > > > > /* The first 64-bit word is the number of subsequent
> > > > > > PCs. */ @@ -568,6 +577,7 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
> > > > > > int mode, i;
> > > > > > struct kcov_remote_arg *remote_arg;
> > > > > > struct kcov_remote *remote;
> > > > > > + struct kcov_pc_range *pc_range;
> > > > > > unsigned long flags;
> > > > > >
> > > > > > switch (cmd) {
> > > > > > @@ -589,6 +599,14 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
> > > > > > kcov->size = size;
> > > > > > kcov->mode = KCOV_MODE_INIT;
> > > > > > return 0;
> > > > > > + case KCOV_PC_RANGE:
> > > > > > + /* Limit trace pc range. */
> > > > > > + pc_range = (struct kcov_pc_range *)arg;
> > > > > > + if (copy_from_user(&kcov->pc_range, pc_range, sizeof(kcov->pc_range)))
> > > > > > + return -EINVAL;
> > > > > > + if (kcov->pc_range.start >= kcov->pc_range.end)
> > > > > > + return -EINVAL;
> > > > > > + return 0;
> > > > > > case KCOV_ENABLE:
> > > > > > /*
> > > > > > * Enable coverage for the current task.
> > > > > > --
> > > > > > 2.7.4

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