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SubjectRe: [RFC PATCH 0/4] tracing/probes: uaccess: Add support user-space access
On Tue, 26 Feb 2019 16:38:50 -0500
Joel Fernandes <joel@joelfernandes.org> wrote:

> On Mon, Feb 25, 2019 at 11:04:42PM +0900, Masami Hiramatsu wrote:

> > Note that kprobe event provides these methods, but it doesn't
> > change it from kernel to user automatically because we do not
> > know whether the given address is in userspace or kernel on
> > some arch.
> > Moreover, from perf-probe, at this moment it is not able to
> > switch. Since __user is not for compiler but checker, we have
> > no clue which data structure is in user-space, in debuginfo.
> >
> > BTW, according to Linus's comment, I implemented probe_user_read()
> > and strncpy_from_unsafe_user() APIs. And since those use
> > "access_ok()" inside it, if CONFIG_DEBUG_ATOMIC_SLEEP=y on x86,
> > it will get a warn message at once. It should be solved before
> > merging this series.
>
> I was wondering why access_ok() can sleep. In the arm64 and x86
> implementation, I don't see access_ok() itself causing a user pointer
> dereference access that can cause a page fault. It seems to just be checking
> the validity of the ranges.
>
> Any idea why the access_ok() code has these comments?
> "Context: User context only. This function may sleep if pagefaults are
> enabled."

Because access_ok() is used only for preparing accessing user-space,
and the user-space access may cause page-fault and sleep.

IMHO, checking in access_ok() inside it is reasonable, but as it
commented, it is for "if pagefaults are enabled.". What we need another
access_ok() for the case when pagefaults are disabled, that is what PeterZ
suggested in below mail.

https://lore.kernel.org/lkml/20190225150603.GE32494@hirez.programming.kicks-ass.net/T/#u


Thank you,

>
> My _guess_ is this is because whatever calls access_ok() may also call
> something else that *does* fault next, if that's the case then that
> WARN_ON_IN_IRQ() in access_ok() is fine, but at least I guess the comments
> should be more clear that it is not access_ok() itself that sleeps.
>
> thanks for any help on understanding this,
>
> - Joel
>
>
> >
> > Thank you,
> >
> > ---
> >
> > Masami Hiramatsu (4):
> > uaccess: Make sure kernel_uaccess_faults_ok is updated before pagefault
> > uaccess: Add non-pagefault user-space read functions
> > tracing/probe: Add ustring type for user-space string
> > tracing/probe: Support user-space dereference
> >
> >
> > Documentation/trace/kprobetrace.rst | 13 ++-
> > Documentation/trace/uprobetracer.rst | 9 +-
> > fs/namespace.c | 2
> > include/linux/uaccess.h | 13 +++
> > kernel/trace/trace.c | 7 +-
> > kernel/trace/trace_kprobe.c | 65 ++++++++++++++++
> > kernel/trace/trace_probe.c | 39 ++++++++--
> > kernel/trace/trace_probe.h | 3 +
> > kernel/trace/trace_probe_tmpl.h | 36 +++++++--
> > kernel/trace/trace_uprobe.c | 19 +++++
> > mm/maccess.c | 138 ++++++++++++++++++++++++++++++----
> > 11 files changed, 302 insertions(+), 42 deletions(-)
> >
> > --
> > Masami Hiramatsu (Linaro) <mhiramat@kernel.org>


--
Masami Hiramatsu <mhiramat@kernel.org>

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