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Date: Mon, 12 Oct 2020 15:12:38 +0100
From: Szabolcs Nagy <szabolcs.nagy@....com>
To: Dave Martin <Dave.Martin@....com>
Cc: "H.J. Lu" <hjl.tools@...il.com>, libc-alpha@...rceware.org,
libc-coord@...ts.openwall.com
Subject: Re: [PATCH] sysconf: Add _SC_MINSIGSTKSZ/_SC_SIGSTKSZ [BZ #20305]
The 10/12/2020 12:04, Dave Martin wrote:
> On Mon, Oct 12, 2020 at 08:53:32AM +0100, Szabolcs Nagy via Libc-alpha wrote:
> > The 10/10/2020 05:19, H.J. Lu via Libc-alpha wrote:
> > > Add _SC_MINSIGSTKSZ for the minimum signal stack size derived from
> > > AT_MINSIGSTKSZ, which is the minimum number of bytes of free stack
> > > space required in order to gurantee successful, non-nested handling
> > > of a single signal whose handler is an empty function, and _SC_SIGSTKSZ
> > > which is the suggested minimum number of bytes of stack space required
> > > for a signal stack.
> > >
> > > If AT_MINSIGSTKSZ isn't available, sysconf (_SC_MINSIGSTKSZ) returns
> > > MINSIGSTKSZ. On Linux/x86 with XSAVE, the signal frame used by kernel
> > > is composed of the following areas and laid out as:
> > >
> > > ------------------------------
> > > | alignment padding |
> > > ------------------------------
> > > | xsave buffer |
> > > ------------------------------
> > > | fsave header (32-bit only) |
> > > ------------------------------
> > > | siginfo + ucontext |
> > > ------------------------------
> > >
> > > Compute AT_MINSIGSTKSZ value as size of xsave buffer + size of fsave
> > > header (32-bit only) + size of siginfo and ucontext + alignment padding.
> > >
> > > If _SC_SIGSTKSZ_SOURCE is defined, MINSIGSTKSZ and SIGSTKSZ are redefined
> > > as
> > >
> > > /* Default stack size for a signal handler: sysconf (SC_SIGSTKSZ). */
> > > # undef SIGSTKSZ
> > > # define SIGSTKSZ sysconf (_SC_SIGSTKSZ)
> > >
> > > /* Minimum stack size for a signal handler: SIGSTKSZ. */
> > > # undef MINSIGSTKSZ
> > > # define MINSIGSTKSZ SIGSTKSZ
> > >
> > > Compilation will fail if the source assumes constant MINSIGSTKSZ or
> > > SIGSTKSZ.
> > >
> > > The reason for not simply increasing the kernel's MINSIGSTKSZ #define
> > > (apart from the fact that it is rarely used, due to glibc's shadowing
> > > definitions) was that userspace binaries will have baked in the old
> > > value of the constant and may be making assumptions about it.
> > >
> > > For example, the type (char [MINSIGSTKSZ]) changes if this #define
> > > changes. This could be a problem if an newly built library tries to
> > > memcpy() or dump such an object defined by and old binary.
> > > Bounds-checking and the stack sizes passed to things like sigaltstack()
> > > and makecontext() could similarly go wrong.
> >
> >
> > this looks reasonable to me.
> >
> > i added libc-coord on cc as it seems to be
> > a useful generic api across targets.
> >
> >
> > /* Return MAX (MINSIGSTKSZ, sysconf (_SC_MINSIGSTKSZ)) * 4. */
> > this can be excessive for sigstksz,
> > but reasonable on glibc given the
> > overhead of libc internal signals
> > and lazy binding.
>
> Interesting points. Can we put actual numbers on those?
>
> Code that tries to allocate correctly sized stacks would need this
> information.
that's not easy in general (and the user would
also need to know the stack use of libc calls).
i think libc internal sigframe and lazy binding
are both roughly MINSIGSTKSZ (because they are
dominated by the register state that has to be
saved), i actually don't know if libc internal
signals go to the user registered sigaltstack
or not (if not then that's not an issue here,
but still an issue for PTHREAD_STACK_MIN).
(if libc internal signals could somehow always
go to a libc internal sigaltstack that would be
best probably, but there is no api for that)
> > does this decrease the size on any
> > existing target?
>
> To avoid unpleasant surprises, I think we should explicitly clamp both
> parameters to be no less than the value of the legacy #define. Then
> the answer becomes "no" by construction. Allowing them to be smaller
> will likely save little memory, so it's probably not worth the risk it.
>
> (The same rule doesn't apply to AT_MINSIGSTKSZ though, since that's a
> new invention, and purely a property of the kernel. We yield the "true"
> kernel value there, which can be less then MINSIGSTKSZ.)
note that there is a posix conformance requirement
on sigaltstack:
The sigaltstack() function shall fail if:
[ENOMEM]
The size of the alternate stack area is less than MINSIGSTKSZ.
if there are multiple different views on what
MINSIGSTKSZ is then it's not clear what this
requirement is about. i think glibc does not
do any checks and leaves it to the kernel, but
that may not be right if there is different
MINSIGSTKSZ in userspace. and if libc tries to
enforce a larger MINSIGSTKSZ than linux then
using AT_MINSIGSTKSZ may unexpectedly fail.
it is probably best not to enforce the failure
in userspace for now, but that does not seem
conforming.
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