Re: [PATCH 2/2] sigaction.2: wfix - Clarify si_addr description.

[Stefan Puiu <stefan.puiu@gmail.com>]

On Thu, Mar 11, 2021 at 7:33 PM Yu, Yu-cheng <yu-cheng.yu@intel.com> wrote:
>
> On 3/11/2021 9:17 AM, Stefan Puiu wrote:
> > Hi,
> >
> > My 2 cents below.
> >
> > On Tue, Mar 9, 2021, 16:33 Borislav Petkov <bp@alien8.de
> > <mailto:bp@alien8.de>> wrote:
> >
> >     On Mon, Mar 08, 2021 at 01:46:07PM -0800, Yu, Yu-cheng wrote:
> >      > I think the sentence above is vague, but probably for the reason
> >     that each
> >      > arch is different.  Maybe this patch is unnecessary and can be
> >     dropped?
> >
> >     Maybe.
> >
> >     If you want to clarify it, you should audit every arch. But what
> >     would that bring? IOW, is it that important to specify when si_addr
> >     is populated and when not...? I don't know of an example but I'm
> >     no userspace programmer anyway, to know when this info would be
> >     beneficial...
> >
> >
> > I've worked on projects where the SIGSEGV sig handler would also print
> > si_addr. When diagnosing a crash, the address that triggered the fault
> > is useful to know. If you can't reproduce the crash in a debugger, or
> > there's no core dump, at least you have an idea if it's a NULL pointer
> > dereference or some naked pointer dereferencing. So I think it's useful
> > to know when si_addr can be used to infer such information and when not.
>
> At least for x86, the faulting ip is already in ucontext, and si_addr is
> mostly the memory address being accessed if that was the reason of the
> fault (i.e. the memory is not supposed to be accessed).  That way, the
> signal handler has both the instruction pointer and the memory address.

Interesting that you mention ucontext. I think the ability to fetch
the IP from it is not that well documented. See for example the
sigaction man page
(https://man7.org/linux/man-pages/man2/sigaction.2.html):

              Further information about the ucontext_t structure can be
              found in getcontext(3) and signal(7).  Commonly, the
              handler function doesn't make any use of the third
              argument.

Michael's book ("The Linux Programming Interface") has similar text on
ucontext ("This information is rarely used in signal handlers, so we
don’t go into further details"). I could find one example on google
for fetching the IP at
https://www.oracle.com/technical-resources/articles/it-infrastructure/dev-signal-handlers-studio.html,
but it pertains to SPARC. Also I've found one older of our projects
that uses this, and it seems each architecture has its own layout (the
code handles ppc, mips and x86-64). Is this documented somewhere?
Outside of the arch-specific kernel definition of the uc_mcontext
member in the code, I mean :).

Thanks,
Stefan.

>
> For shadow stack violation, for example, it is not because the memory
> being accessed; it is the instruction itself causing the violation.  It
> is unnecessary to duplicate the ip in si_addr.  Setting si_addr to zero
> also indicates this is not a memory type fault.
>
> --
> Yu-cheng