On Tue, Oct 6, 2020 at 8:43 AM Dave Martin <Dave.Martin@xxxxxxx> wrote: > > On Tue, Oct 06, 2020 at 08:18:03AM -0700, H.J. Lu wrote: > > On Tue, Oct 6, 2020 at 5:12 AM H.J. Lu <hjl.tools@xxxxxxxxx> wrote: > > > > > > On Tue, Oct 6, 2020 at 2:25 AM Dave Martin <Dave.Martin@xxxxxxx> wrote: > > > > > > > > On Mon, Oct 05, 2020 at 10:17:06PM +0100, H.J. Lu wrote: > > > > > On Mon, Oct 5, 2020 at 6:45 AM Dave Martin <Dave.Martin@xxxxxxx> wrote: > > > > > > > > > > > > On Tue, Sep 29, 2020 at 01:57:42PM -0700, Chang S. Bae wrote: > > > > > > > During signal entry, the kernel pushes data onto the normal userspace > > > > > > > stack. On x86, the data pushed onto the user stack includes XSAVE state, > > > > > > > which has grown over time as new features and larger registers have been > > > > > > > added to the architecture. > > > > > > > > > > > > > > MINSIGSTKSZ is a constant provided in the kernel signal.h headers and > > > > > > > typically distributed in lib-dev(el) packages, e.g. [1]. Its value is > > > > > > > compiled into programs and is part of the user/kernel ABI. The MINSIGSTKSZ > > > > > > > constant indicates to userspace how much data the kernel expects to push on > > > > > > > the user stack, [2][3]. > > > > > > > > > > > > > > However, this constant is much too small and does not reflect recent > > > > > > > additions to the architecture. For instance, when AVX-512 states are in > > > > > > > use, the signal frame size can be 3.5KB while MINSIGSTKSZ remains 2KB. > > > > > > > > > > > > > > The bug report [4] explains this as an ABI issue. The small MINSIGSTKSZ can > > > > > > > cause user stack overflow when delivering a signal. > > > > > > > > > > > > > > In this series, we suggest a couple of things: > > > > > > > 1. Provide a variable minimum stack size to userspace, as a similar > > > > > > > approach to [5] > > > > > > > 2. Avoid using a too-small alternate stack > > > > > > > > > > > > I can't comment on the x86 specifics, but the approach followed in this > > > > > > series does seem consistent with the way arm64 populates > > > > > > AT_MINSIGSTKSZ. > > > > > > > > > > > > I need to dig up my glibc hacks for providing a sysconf interface to > > > > > > this... > > > > > > > > > > Here is my proposal for glibc: > > > > > > > > > > https://sourceware.org/pipermail/libc-alpha/2020-September/118098.html > > > > > > > > Thanks for the link. > > > > > > > > Are there patches yet? I already had some hacks in the works, but I can > > > > drop them if there's something already out there. > > > > > > I am working on it. > > > > > > > > > > > > 1. Define SIGSTKSZ and MINSIGSTKSZ to 64KB. > > > > > > > > Can we do this? IIUC, this is an ABI break and carries the risk of > > > > buffer overruns. > > > > > > > > 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. > > > > > > With my original proposal: > > > > > > https://sourceware.org/pipermail/libc-alpha/2020-September/118028.html > > > > > > char [MINSIGSTKSZ] won't compile. The feedback is to increase the > > > constants: > > > > > > https://sourceware.org/pipermail/libc-alpha/2020-September/118092.html > > > > > > > > > > > > 2. Add _SC_RSVD_SIG_STACK_SIZE for signal stack size reserved by the kernel. > > > > > > > > How about "_SC_MINSIGSTKSZ"? This was my initial choice since only the > > > > discovery method is changing. The meaning of the value is exactly the > > > > same as before. > > > > > > > > If we are going to rename it though, it could make sense to go for > > > > something more directly descriptive, say, "_SC_SIGNAL_FRAME_SIZE". > > > > > > > > The trouble with including "STKSZ" is that is sounds like a > > > > recommendation for your stack size. While the signal frame size is > > > > relevant to picking a stack size, it's not the only thing to > > > > consider. > > > > > > The problem is that AT_MINSIGSTKSZ is the signal frame size used by > > > kernel. The minimum stack size for a signal handler is more likely > > > AT_MINSIGSTKSZ + 1.5KB unless AT_MINSIGSTKSZ returns the signal > > > frame size used by kernel + 6KB for user application. > > > > > > > > > > > Also, do we need a _SC_SIGSTKSZ constant, or should the entire concept > > > > of a "recommended stack size" be abandoned? glibc can at least make a > > > > slightly more informed guess about suitable stack sizes than the kernel > > > > (and glibc already has to guess anyway, in order to determine the > > > > default thread stack size). > > > > > > Glibc should try to deduct signal frame size if AT_MINSIGSTKSZ isn't > > > available. > > > > > > > > > > > > 3. Deprecate SIGSTKSZ and MINSIGSTKSZ if _SC_RSVD_SIG_STACK_SIZE > > > > > is in use. > > > > > > > > Great if we can do it. I was concerned that this might be > > > > controversial. > > > > > > > > Would this just be a recommendation, or can we enforce it somehow? > > > > > > It is just an idea. We need to move away from constant SIGSTKSZ and > > > MINSIGSTKSZ. > > > > > > > Here is the glibc patch: > > > > https://gitlab.com/x86-glibc/glibc/-/commits/users/hjl/AT_MINSIGSTKSZ > > > > AT_MINSIGSTKSZ should return the signal frame size used by kernel + 6KB > > for user application. > > I'm not sure about the 6K here. 6KB is something I made up. > We a few fundamental parameters: > > * the actual maximum size of the kernel-allocated signal frame (which > we'll report via AT_MINSIGSTKSZ); Agree. > * the size of additional userspace stack frame required to execute the > minimal (i.e., empty) signal handler. (On AArch64, this is 0. In It is also 0 for x86. > environments where the C lirbrary calls signal handlers through some > sort of wrapper, this would need to include the wrapper's stack > needs also); > * additional userspace stack needs for the actual signal handler code. > This is completely unknown. That is 6KB I made up. > > _SC_MINSIGSTKSZ (however named) should certainly include the first two, > but I'm not sure about the third. It will at least be architecture- > dependent. > > > This is one reason why I still favor having more than one constant here: > the fundamental system properties should be discoverable for software > that knows how to calculate its own stack needs accurately. > > Since calculating stack needs is hard and most software doesn't bother > to do it, we could also give a "recommended" stack size which > incorporates a guess of typical handler stack needs (similarly to the > legacy SIGSTKSZ constant), but I think that should be a separate > parameter. Sounds reasonable. We can have _SC_MINSIGSTKSZ and _SC_SIGSTKSZ which is _SC_MINSIGSTKSZ + 6KB (or some other value). -- H.J.
