Hi Josh (and CC:lkml),
I've recently transitioned to an Arch system which has
CONFIG_ORC_UNWINDER=y in the default kernel. My window manager
integrates process monitoring showing the wchans of processes, making
it very apparent when wchan breaks.
Glancing at the kernel code to see what's involved in get_wchan() for
x86, it looks to assume there are frame pointers in the stack. I
don't see any mention of ORC_UNWINDER in the get_wchan() code which
seems like an oversight when ORC_UNWINDER=y gets rid of them.
I had originally assumed this was just a Kconfig problem and asked
lkml about it (hearing crickets back) [0], but have since learned of
ORC_UNWINDER's existence via the Arch kernel maintainer.
Is this an oversight of the ORC_UNWINDER implementation? It's
arguably a regression to completely break wchans for tools like `ps -o
wchan` and `top`, or my window manager and its separate monitoring
utility. Presumably there are other tools out there sampling wchans
for monitoring as well, there's also an internal use of get_chan() in
kernel/sched/fair.c for sleep profiling.
I've occasionally seen when monitoring at a high sample rate (60hz) on
something churny like a parallel kernel or systemd build, there's a
spurious non-zero sample coming out of /proc/[pid]/wchan containing a
hexadecimal address like 0xffffa9ebc181bcf8. This all smells broken,
is get_wchan() occasionally spitting out random junk here kallsyms
can't resolve, because get_chan() is completely ignorant of
ORC_UNWINDER's effects?
My time to spend on this currently is very limited, but I'd like to at
least get the relevant parties aware if they're not already... Maybe
I should just file something in bugzilla.
Thanks,
Vito Caputo
[0] https://lore.kernel.org/lkml/[email protected]/
On Tue, Sep 21, 2021 at 12:32:49PM -0700, Vito Caputo wrote:
> Is this an oversight of the ORC_UNWINDER implementation? It's
> arguably a regression to completely break wchans for tools like `ps -o
> wchan` and `top`, or my window manager and its separate monitoring
> utility. Presumably there are other tools out there sampling wchans
> for monitoring as well, there's also an internal use of get_chan() in
> kernel/sched/fair.c for sleep profiling.
>
> I've occasionally seen when monitoring at a high sample rate (60hz) on
> something churny like a parallel kernel or systemd build, there's a
> spurious non-zero sample coming out of /proc/[pid]/wchan containing a
> hexadecimal address like 0xffffa9ebc181bcf8. This all smells broken,
> is get_wchan() occasionally spitting out random junk here kallsyms
> can't resolve, because get_chan() is completely ignorant of
> ORC_UNWINDER's effects?
Hi Vito,
Thanks for reporting this. Does this patch fix your issue?
https://lkml.kernel.org/r/[email protected]
Though, considering wchan has been silently broken for four years, I do
wonder what the impact would be if we were to just continue to show "0"
(and change frame pointers to do the same).
The kernel is much more cautious than it used to be about exposing this
type of thing. Can you elaborate on your use case?
If we do keep it, we might want to require CAP_SYS_ADMIN anyway, for
similar reasons as
f8a00cef1720 ("proc: restrict kernel stack dumps to root")
... since presumably proc_pid_wchan()'s use of '%ps' can result in an
actual address getting printed if the unwind gets confused, thanks to
__sprint_symbol()'s backup option if kallsyms_lookup_buildid() doesn't
find a name.
Though, instead of requiring CAP_SYS_ADMIN, maybe we can just fix
__sprint_symbol() to not expose addresses?
Or is there some other reason for needing CAP_SYS_ADMIN? Jann?
--
Josh
On Tue, Sep 21, 2021 at 05:15:37PM -0700, Josh Poimboeuf wrote:
> On Tue, Sep 21, 2021 at 12:32:49PM -0700, Vito Caputo wrote:
> > Is this an oversight of the ORC_UNWINDER implementation? It's
> > arguably a regression to completely break wchans for tools like `ps -o
> > wchan` and `top`, or my window manager and its separate monitoring
> > utility. Presumably there are other tools out there sampling wchans
> > for monitoring as well, there's also an internal use of get_chan() in
> > kernel/sched/fair.c for sleep profiling.
> >
> > I've occasionally seen when monitoring at a high sample rate (60hz) on
> > something churny like a parallel kernel or systemd build, there's a
> > spurious non-zero sample coming out of /proc/[pid]/wchan containing a
> > hexadecimal address like 0xffffa9ebc181bcf8. This all smells broken,
> > is get_wchan() occasionally spitting out random junk here kallsyms
> > can't resolve, because get_chan() is completely ignorant of
> > ORC_UNWINDER's effects?
>
> Hi Vito,
>
> Thanks for reporting this. Does this patch fix your issue?
>
> https://lkml.kernel.org/r/[email protected]
>
> Though, considering wchan has been silently broken for four years, I do
> wonder what the impact would be if we were to just continue to show "0"
> (and change frame pointers to do the same).
>
> The kernel is much more cautious than it used to be about exposing this
> type of thing. Can you elaborate on your use case?
>
> If we do keep it, we might want to require CAP_SYS_ADMIN anyway, for
> similar reasons as
>
> f8a00cef1720 ("proc: restrict kernel stack dumps to root")
Normally wchan is protected by:
ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS)
I might argue that this check isn't right -- it needs to be using
f_cred, but I'll let Jann answer more there.
> ... since presumably proc_pid_wchan()'s use of '%ps' can result in an
> actual address getting printed if the unwind gets confused, thanks to
> __sprint_symbol()'s backup option if kallsyms_lookup_buildid() doesn't
> find a name.
Ew, yeah, __sprint_symbol() falls back to exposing addresses. :(
name = kallsyms_lookup_buildid(address, &size, &offset, &modname, &buildid,
buffer);
if (!name)
return sprintf(buffer, "0x%lx", address - symbol_offset);
Thought I can't immediately think of what wouldn't be symbolized by
kallsyms_lookup_buildid(), but given it fails open, I can totally
believe there is. :)
is_ksym_addr()
module_address_lookup()
bpf_address_lookup()
ftrace_mod_address_lookup()
> Though, instead of requiring CAP_SYS_ADMIN, maybe we can just fix
> __sprint_symbol() to not expose addresses?
>
> Or is there some other reason for needing CAP_SYS_ADMIN? Jann?
While it's not very high fidelity, I don't like having the kernel
symbols exposed like this because userspace can basically sample the
execution path of syscalls, etc. It's not a raw value, but it still
creeps me out given that it can be probed.
So, if it's been broken for 4 years under ORC, how about we just disable
wchan permanently? (Untested...)
diff --git a/fs/proc/array.c b/fs/proc/array.c
index 49be8c8ef555..cfa60e22a5de 100644
--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -452,7 +452,7 @@ int proc_pid_status(struct seq_file *m, struct pid_namespace *ns,
static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
struct pid *pid, struct task_struct *task, int whole)
{
- unsigned long vsize, eip, esp, wchan = 0;
+ unsigned long vsize, eip, esp = 0;
int priority, nice;
int tty_pgrp = -1, tty_nr = 0;
sigset_t sigign, sigcatch;
@@ -540,8 +540,6 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
unlock_task_sighand(task, &flags);
}
- if (permitted && (!whole || num_threads < 2))
- wchan = get_wchan(task);
if (!whole) {
min_flt = task->min_flt;
maj_flt = task->maj_flt;
@@ -600,16 +598,10 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
seq_put_decimal_ull(m, " ", sigcatch.sig[0] & 0x7fffffffUL);
/*
- * We used to output the absolute kernel address, but that's an
- * information leak - so instead we show a 0/1 flag here, to signal
- * to user-space whether there's a wchan field in /proc/PID/wchan.
- *
- * This works with older implementations of procps as well.
+ * We used to output the absolute kernel address, and then just
+ * a symbol. But both are information leaks.
*/
- if (wchan)
- seq_puts(m, " 1");
- else
- seq_puts(m, " 0");
+ seq_puts(m, " 0");
seq_put_decimal_ull(m, " ", 0);
seq_put_decimal_ull(m, " ", 0);
diff --git a/fs/proc/base.c b/fs/proc/base.c
index 533d5836eb9a..52484cd77f99 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -378,24 +378,10 @@ static const struct file_operations proc_pid_cmdline_ops = {
};
#ifdef CONFIG_KALLSYMS
-/*
- * Provides a wchan file via kallsyms in a proper one-value-per-file format.
- * Returns the resolved symbol. If that fails, simply return the address.
- */
static int proc_pid_wchan(struct seq_file *m, struct pid_namespace *ns,
struct pid *pid, struct task_struct *task)
{
- unsigned long wchan;
-
- if (ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS))
- wchan = get_wchan(task);
- else
- wchan = 0;
-
- if (wchan)
- seq_printf(m, "%ps", (void *) wchan);
- else
- seq_putc(m, '0');
+ seq_putc(m, '0');
return 0;
}
If not that, then we need to fix the fallback: (Also untested...)
diff --git a/fs/proc/base.c b/fs/proc/base.c
index 533d5836eb9a..0f228c421e5f 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -393,7 +393,7 @@ static int proc_pid_wchan(struct seq_file *m, struct pid_namespace *ns,
wchan = 0;
if (wchan)
- seq_printf(m, "%ps", (void *) wchan);
+ seq_printf(m, "%pSf", (void *) wchan);
else
seq_putc(m, '0');
diff --git a/include/linux/kallsyms.h b/include/linux/kallsyms.h
index a1d6fc82d7f0..8cce2b5c36df 100644
--- a/include/linux/kallsyms.h
+++ b/include/linux/kallsyms.h
@@ -96,6 +96,7 @@ const char *kallsyms_lookup(unsigned long addr,
extern int sprint_symbol(char *buffer, unsigned long address);
extern int sprint_symbol_build_id(char *buffer, unsigned long address);
extern int sprint_symbol_no_offset(char *buffer, unsigned long address);
+extern int sprint_symbol_no_fallback(char *buffer, unsigned long address);
extern int sprint_backtrace(char *buffer, unsigned long address);
extern int sprint_backtrace_build_id(char *buffer, unsigned long address);
diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
index 0ba87982d017..06ebd27dcd3a 100644
--- a/kernel/kallsyms.c
+++ b/kernel/kallsyms.c
@@ -418,7 +418,8 @@ int lookup_symbol_attrs(unsigned long addr, unsigned long *size,
/* Look up a kernel symbol and return it in a text buffer. */
static int __sprint_symbol(char *buffer, unsigned long address,
- int symbol_offset, int add_offset, int add_buildid)
+ int symbol_offset, int add_offset, int add_buildid,
+ int fallback)
{
char *modname;
const unsigned char *buildid;
@@ -429,8 +430,11 @@ static int __sprint_symbol(char *buffer, unsigned long address,
address += symbol_offset;
name = kallsyms_lookup_buildid(address, &size, &offset, &modname, &buildid,
buffer);
- if (!name)
+ if (!name) {
+ if (!fallback)
+ return sprintf(buffer, "0");
return sprintf(buffer, "0x%lx", address - symbol_offset);
+ }
if (name != buffer)
strcpy(buffer, name);
@@ -470,7 +474,7 @@ static int __sprint_symbol(char *buffer, unsigned long address,
*/
int sprint_symbol(char *buffer, unsigned long address)
{
- return __sprint_symbol(buffer, address, 0, 1, 0);
+ return __sprint_symbol(buffer, address, 0, 1, 0, 1);
}
EXPORT_SYMBOL_GPL(sprint_symbol);
@@ -487,7 +491,7 @@ EXPORT_SYMBOL_GPL(sprint_symbol);
*/
int sprint_symbol_build_id(char *buffer, unsigned long address)
{
- return __sprint_symbol(buffer, address, 0, 1, 1);
+ return __sprint_symbol(buffer, address, 0, 1, 1, 1);
}
EXPORT_SYMBOL_GPL(sprint_symbol_build_id);
@@ -504,10 +508,27 @@ EXPORT_SYMBOL_GPL(sprint_symbol_build_id);
*/
int sprint_symbol_no_offset(char *buffer, unsigned long address)
{
- return __sprint_symbol(buffer, address, 0, 0, 0);
+ return __sprint_symbol(buffer, address, 0, 0, 0, 1);
}
EXPORT_SYMBOL_GPL(sprint_symbol_no_offset);
+/**
+ * sprint_symbol_no_fallback - Look up a kernel symbol and return it in a text buffer
+ * @buffer: buffer to be stored
+ * @address: address to lookup
+ *
+ * This function looks up a kernel symbol with @address and stores its name
+ * and module name to @buffer if possible. If no symbol was found, returns
+ * "0".
+ *
+ * This function returns the number of bytes stored in @buffer.
+ */
+int sprint_symbol_no_fallback(char *buffer, unsigned long address)
+{
+ return __sprint_symbol(buffer, address, 0, 0, 0, 0);
+}
+EXPORT_SYMBOL_GPL(sprint_symbol_no_fallback);
+
/**
* sprint_backtrace - Look up a backtrace symbol and return it in a text buffer
* @buffer: buffer to be stored
@@ -524,7 +545,7 @@ EXPORT_SYMBOL_GPL(sprint_symbol_no_offset);
*/
int sprint_backtrace(char *buffer, unsigned long address)
{
- return __sprint_symbol(buffer, address, -1, 1, 0);
+ return __sprint_symbol(buffer, address, -1, 1, 0, 1);
}
/**
@@ -544,7 +565,7 @@ int sprint_backtrace(char *buffer, unsigned long address)
*/
int sprint_backtrace_build_id(char *buffer, unsigned long address)
{
- return __sprint_symbol(buffer, address, -1, 1, 1);
+ return __sprint_symbol(buffer, address, -1, 1, 1, 1);
}
/* To avoid using get_symbol_offset for every symbol, we carry prefix along. */
diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index d7ad44f2c8f5..83c1065da996 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -999,6 +999,8 @@ char *symbol_string(char *buf, char *end, void *ptr,
sprint_backtrace(sym, value);
else if (*fmt == 'S' && (fmt[1] == 'b' || (fmt[1] == 'R' && fmt[2] == 'b')))
sprint_symbol_build_id(sym, value);
+ else if (*fmt == 'S' && fmt[1] == 'f')
+ sprint_symbol_no_fallback(sym, value);
else if (*fmt != 's')
sprint_symbol(sym, value);
else
@@ -2268,6 +2270,8 @@ early_param("no_hash_pointers", no_hash_pointers_enable);
* - 's' For symbolic direct pointers (or function descriptors) without offset
* - '[Ss]R' as above with __builtin_extract_return_addr() translation
* - 'S[R]b' as above with module build ID (for use in backtraces)
+ * - 'Sf' For symbolic direct pointers (or function descriptors) without offset
+ * without fallback to unsymbolized address.
* - '[Ff]' %pf and %pF were obsoleted and later removed in favor of
* %ps and %pS. Be careful when re-using these specifiers.
* - 'B' For backtraced symbolic direct pointers with offset
--
Kees Cook
On 9/22/21 8:15 AM, Josh Poimboeuf wrote:
> On Tue, Sep 21, 2021 at 12:32:49PM -0700, Vito Caputo wrote:
>> Is this an oversight of the ORC_UNWINDER implementation? It's
>> arguably a regression to completely break wchans for tools like `ps -o
>> wchan` and `top`, or my window manager and its separate monitoring
>> utility. Presumably there are other tools out there sampling wchans
>> for monitoring as well, there's also an internal use of get_chan() in
>> kernel/sched/fair.c for sleep profiling.
>>
>> I've occasionally seen when monitoring at a high sample rate (60hz) on
>> something churny like a parallel kernel or systemd build, there's a
>> spurious non-zero sample coming out of /proc/[pid]/wchan containing a
>> hexadecimal address like 0xffffa9ebc181bcf8. This all smells broken,
>> is get_wchan() occasionally spitting out random junk here kallsyms
>> can't resolve, because get_chan() is completely ignorant of
>> ORC_UNWINDER's effects?
>
> Hi Vito,
>
> Thanks for reporting this. Does this patch fix your issue?
>
> https://lkml.kernel.org/r/[email protected]
>
> Though, considering wchan has been silently broken for four years, I do
> wonder what the impact would be if we were to just continue to show "0"
> (and change frame pointers to do the same).
Agree, Or remove get_wchan() directly.
>
> The kernel is much more cautious than it used to be about exposing this
> type of thing. Can you elaborate on your use case?
>
> If we do keep it, we might want to require CAP_SYS_ADMIN anyway, for
> similar reasons as
>
> f8a00cef1720 ("proc: restrict kernel stack dumps to root")
>
> ... since presumably proc_pid_wchan()'s use of '%ps' can result in an
> actual address getting printed if the unwind gets confused, thanks to
> __sprint_symbol()'s backup option if kallsyms_lookup_buildid() doesn't
> find a name.
>
> Though, instead of requiring CAP_SYS_ADMIN, maybe we can just fix
> __sprint_symbol() to not expose addresses?
>
> Or is there some other reason for needing CAP_SYS_ADMIN? Jann?
>
On Wed, Sep 22, 2021 at 5:05 AM Kees Cook <[email protected]> wrote:
> On Tue, Sep 21, 2021 at 05:15:37PM -0700, Josh Poimboeuf wrote:
> > On Tue, Sep 21, 2021 at 12:32:49PM -0700, Vito Caputo wrote:
> > > Is this an oversight of the ORC_UNWINDER implementation? It's
> > > arguably a regression to completely break wchans for tools like `ps -o
> > > wchan` and `top`, or my window manager and its separate monitoring
> > > utility. Presumably there are other tools out there sampling wchans
> > > for monitoring as well, there's also an internal use of get_chan() in
> > > kernel/sched/fair.c for sleep profiling.
> > >
> > > I've occasionally seen when monitoring at a high sample rate (60hz) on
> > > something churny like a parallel kernel or systemd build, there's a
> > > spurious non-zero sample coming out of /proc/[pid]/wchan containing a
> > > hexadecimal address like 0xffffa9ebc181bcf8. This all smells broken,
> > > is get_wchan() occasionally spitting out random junk here kallsyms
> > > can't resolve, because get_chan() is completely ignorant of
> > > ORC_UNWINDER's effects?
> >
> > Hi Vito,
> >
> > Thanks for reporting this. Does this patch fix your issue?
> >
> > https://lkml.kernel.org/r/[email protected]
> >
> > Though, considering wchan has been silently broken for four years, I do
> > wonder what the impact would be if we were to just continue to show "0"
> > (and change frame pointers to do the same).
> >
> > The kernel is much more cautious than it used to be about exposing this
> > type of thing. Can you elaborate on your use case?
> >
> > If we do keep it, we might want to require CAP_SYS_ADMIN anyway, for
> > similar reasons as
> >
> > f8a00cef1720 ("proc: restrict kernel stack dumps to root")
>
> Normally wchan is protected by:
>
> ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS)
>
> I might argue that this check isn't right -- it needs to be using
> f_cred, but I'll let Jann answer more there.
>
> > ... since presumably proc_pid_wchan()'s use of '%ps' can result in an
> > actual address getting printed if the unwind gets confused, thanks to
> > __sprint_symbol()'s backup option if kallsyms_lookup_buildid() doesn't
> > find a name.
>
> Ew, yeah, __sprint_symbol() falls back to exposing addresses. :(
>
> name = kallsyms_lookup_buildid(address, &size, &offset, &modname, &buildid,
> buffer);
> if (!name)
> return sprintf(buffer, "0x%lx", address - symbol_offset);
Whaaaat? That's not how wchan worked when I looked at this the last
time a few years ago...
... ah, commit 152c432b128cb ("proc/wchan: use printk format instead
of lookup_symbol_name()") is to blame for that.
> Thought I can't immediately think of what wouldn't be symbolized by
> kallsyms_lookup_buildid(), but given it fails open, I can totally
> believe there is. :)
One thing that makes /proc/$pid/wchan and /proc/$pid/stack so awful is
that they unwind a task's stack without ensuring that that stack isn't
actually running. That's why /proc/$pid/stack got restricted to root
years ago, see <https://bugs.chromium.org/p/project-zero/issues/detail?id=1657>
- I wrote an exploit back then that manages to read a pointer from an
arbitrary stack location via /proc/$pid/stack.
> is_ksym_addr()
> module_address_lookup()
> bpf_address_lookup()
> ftrace_mod_address_lookup()
>
> > Though, instead of requiring CAP_SYS_ADMIN, maybe we can just fix
> > __sprint_symbol() to not expose addresses?
> >
> > Or is there some other reason for needing CAP_SYS_ADMIN? Jann?
>
> While it's not very high fidelity, I don't like having the kernel
> symbols exposed like this because userspace can basically sample the
> execution path of syscalls, etc. It's not a raw value, but it still
> creeps me out given that it can be probed.
>
> So, if it's been broken for 4 years under ORC, how about we just disable
> wchan permanently? (Untested...)
If that's possible, I would like it very much. I don't know whether
there are people on arm64 or so that rely on it though...
On Tue, Sep 21, 2021 at 05:15:37PM -0700, Josh Poimboeuf wrote:
> On Tue, Sep 21, 2021 at 12:32:49PM -0700, Vito Caputo wrote:
> > Is this an oversight of the ORC_UNWINDER implementation? It's
> > arguably a regression to completely break wchans for tools like `ps -o
> > wchan` and `top`, or my window manager and its separate monitoring
> > utility. Presumably there are other tools out there sampling wchans
> > for monitoring as well, there's also an internal use of get_chan() in
> > kernel/sched/fair.c for sleep profiling.
> >
> > I've occasionally seen when monitoring at a high sample rate (60hz) on
> > something churny like a parallel kernel or systemd build, there's a
> > spurious non-zero sample coming out of /proc/[pid]/wchan containing a
> > hexadecimal address like 0xffffa9ebc181bcf8. This all smells broken,
> > is get_wchan() occasionally spitting out random junk here kallsyms
> > can't resolve, because get_chan() is completely ignorant of
> > ORC_UNWINDER's effects?
>
> Hi Vito,
>
> Thanks for reporting this. Does this patch fix your issue?
>
> https://lkml.kernel.org/r/[email protected]
Yes, it fixes things for me, thanks for the quick reply and fix!
>
> Though, considering wchan has been silently broken for four years, I do
> wonder what the impact would be if we were to just continue to show "0"
> (and change frame pointers to do the same).
>
> The kernel is much more cautious than it used to be about exposing this
> type of thing. Can you elaborate on your use case?
>
My personal use case is my window manager continuously monitors all
processes associated with a given window, maintaining CPU utilization
graph overlays at all times, which may be toggled visible at a
keypress.
One of the columns in these overlays is the WCHAN, which helps
understand where processes are blocking and generally how programs are
written/architected. When I switched to using Arch with the included
kernel having ORC_UNWDINER=y, this broke. I had previously been using
Debian for decades, but at some point switched to building my own
kernel with my own .config and never switched to using ORC_UNWINDER.
So despite ORC_UNWINDER being the defconfig for years, I never used it
until recently on the Arch machine.
It's unclear to me how much can be inferred from ORC_UNWINDER breaking
this since 2017, since CONFIG_FRAME_POINTER has preserved the
functionality this whole time and I don't know what
users/distros/cloud providers have continued using that setting,
ignoring ORC_UNWDINER altogether.
Isn't it entirely plausible that others have been doing the same thing
I've been doing with my .config and haven't yet got on the
ORC_UNWINDER train? It's kind of YOLO to just ship the defconfig when
you've got a stable configuration your end-users are relying upon.
> If we do keep it, we might want to require CAP_SYS_ADMIN anyway, for
> similar reasons as
>
> f8a00cef1720 ("proc: restrict kernel stack dumps to root")
>
> ... since presumably proc_pid_wchan()'s use of '%ps' can result in an
> actual address getting printed if the unwind gets confused, thanks to
> __sprint_symbol()'s backup option if kallsyms_lookup_buildid() doesn't
> find a name.
>
> Though, instead of requiring CAP_SYS_ADMIN, maybe we can just fix
> __sprint_symbol() to not expose addresses?
>
> Or is there some other reason for needing CAP_SYS_ADMIN? Jann?
>
I'd really appreciate it if my regular user could continue accessing
the wchan at least for my own processes. My monitoring tooling which
I've invested significant time in are an integral part of my
development workflow. I regularly flip on the overlays to observe
changes during testing and seeing which WCHANs my code is spending
time in at a glance is quite useful. If this is some kind of serious
security problem, then perhaps expose a sysctl to enable users access
their own wchans. It's obviously not an issue for us developers on
non-multiuser linux hosts we're just writing and testing code on.
Thanks,
Vito Caputo
From: Jann Horn
> Sent: 24 September 2021 01:00
...
> > Normally wchan is protected by:
> >
> > ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS)
> >
> > I might argue that this check isn't right -- it needs to be using
> > f_cred, but I'll let Jann answer more there.
> >
> > > ... since presumably proc_pid_wchan()'s use of '%ps' can result in an
> > > actual address getting printed if the unwind gets confused, thanks to
> > > __sprint_symbol()'s backup option if kallsyms_lookup_buildid() doesn't
> > > find a name.
> >
> > Ew, yeah, __sprint_symbol() falls back to exposing addresses. :(
> >
> > name = kallsyms_lookup_buildid(address, &size, &offset, &modname, &buildid,
> > buffer);
> > if (!name)
> > return sprintf(buffer, "0x%lx", address - symbol_offset);
>
> Whaaaat? That's not how wchan worked when I looked at this the last
> time a few years ago...
Hmmmm....
Historically (and I mean SYSV and probably BSD) the 'wchan'
was the 'token' passed to the kernel sleep() function and
value that needed to be passed to wakeup() to get the process
rescheduled.
It was usually the address of something associated with the
sleep - but didn't have to be.
Linux doesn't do process sleep/wakeup the same way.
The nearest thing would be 'struct wait_queue_head'.
But the address of that isn't a useful value.
Plausibly the address of the function that initialises
the wait_queue_head could be put into it and that
value saved by schedule() to that it can be returned
as the 'wchan'.
David
-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
On Tue, Sep 21, 2021, at 8:30 PM, Qi Zheng wrote:
> On 9/22/21 8:15 AM, Josh Poimboeuf wrote:
>> On Tue, Sep 21, 2021 at 12:32:49PM -0700, Vito Caputo wrote:
>>> Is this an oversight of the ORC_UNWINDER implementation? It's
>>> arguably a regression to completely break wchans for tools like `ps -o
>>> wchan` and `top`, or my window manager and its separate monitoring
>>> utility. Presumably there are other tools out there sampling wchans
>>> for monitoring as well, there's also an internal use of get_chan() in
>>> kernel/sched/fair.c for sleep profiling.
>>>
>>> I've occasionally seen when monitoring at a high sample rate (60hz) on
>>> something churny like a parallel kernel or systemd build, there's a
>>> spurious non-zero sample coming out of /proc/[pid]/wchan containing a
>>> hexadecimal address like 0xffffa9ebc181bcf8. This all smells broken,
>>> is get_wchan() occasionally spitting out random junk here kallsyms
>>> can't resolve, because get_chan() is completely ignorant of
>>> ORC_UNWINDER's effects?
>>
>> Hi Vito,
>>
>> Thanks for reporting this. Does this patch fix your issue?
>>
>> https://lkml.kernel.org/r/[email protected]
>>
>> Though, considering wchan has been silently broken for four years, I do
>> wonder what the impact would be if we were to just continue to show "0"
>> (and change frame pointers to do the same).
>
> Agree, Or remove get_wchan() directly.
I agree. wchan is a hack that may or may not do anything useful. We certainly should not be reporting things derived from the stack trace to unprivileged tasks. And it's probably just as racy as /proc/.../stack.