On Tue, Oct 06, 2020 at 08:34:06AM -0700, H.J. Lu wrote: > On Tue, Oct 6, 2020 at 8:25 AM Dave Martin <Dave.Martin@xxxxxxx> wrote: > > > > On Tue, Oct 06, 2020 at 05:12:29AM -0700, H.J. Lu 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. > > > > OK. I may post something for discussion, but I'm happy for it to be > > superseded by someone (i.e., other than me) who actually knows what > > they're doing... > > Please see my previous email for my glibc patch: > > https://gitlab.com/x86-glibc/glibc/-/commits/users/hjl/AT_MINSIGSTKSZ > > > > > > > > > > 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 > > > > Ah, I see. But both still API and ABI breaks; moreover, declaraing an > > array with size based on (MIN)SIGSTKSZ is not just reasonable, but the > > obvious thing to do with this constant in many simple cases. Such usage > > is widespread, see: > > > > * https://codesearch.debian.net/search?q=%5BSIGSTKSZ%5D&literal=1 > > > > > > Your two approaches seem to trade off two different sources of buffer > > overruns: undersized stacks versus ABI breaks across library boundaries. > > We can't get everything we want. > > > Since undersized stack is by far the more familiar problem and we at > > least have guard regions to help detect overruns, I'd vote to keep > > MINSIGSTKSZ and SIGSTKSZ as-is, at least for now. > > Agree. > > > Or are people reporting real stack overruns on x86 today? > > I hope so. > > > > > For arm64, we made large vectors on SVE opt-in, so that oversized signal > > frames are not seen by default. Would somethine similar be feasible on > > x86? > > > > > > > > > 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. > > > > Ack; to be correct, you also need to take into account which signals may > > be unmasked while running on this stack, and the stack requirements of > > all their handlers. Unfortunately, that's hard :( > > > > What's your view on my naming suggesions? > > I used _SC_MINSIGSTKSZ: > > https://gitlab.com/x86-glibc/glibc/-/commit/73ca53bfbc1c105bc579f55f15af011a07fcded9 Apologies, I missed that. Otherwise, the changes look much as I would expect, except for the "6K for user program" thing. This is strictly not included in the legacy MINSIGSTKSZ. > > > > > > > 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. > > > > In my code, I generate _SC_SIGSTKSZ as the equivalent of > > > > max(sysconf(_SC_MINSIGSTKSZ) * 4, SIGSTKSZ) > > > > which is >= the legacy value, and broadly reperesentative of the > > relationship between MINSIGSTKSZ and SIGSTKSZ on most arches. > > > > > > What do you think? > > sysconf(_SC_MINSIGSTKSZ) should be usable ASIS for most cases. Why, though? MINSIGSTKSZ is not specified to be usable as-is for any case whatsoever. Software that calculates its own needs to know the actual system values, not estimates based on guesses about how much stack a typical program might need if it were recompiled for x86. This doesn't mean we can't have a generic suggested value that's suitable for common scenarios (like SIGSTKSZ), but if we do then I think it should be a separate constant. > > > > > 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. > > > > Totally agree with that. > > > > With my glibc patch, -D_SC_MINSIGSTKSZ_SOURCE will fail to compile > if the source assumes constant SIGSTKSZ or MINSIGSTKSZ. Ah yes, I see. That's a sensible precaution. Is it accepted in general that defining different feature test macros can lead to ABI incompatibilities? I have thought that building a shared library with _GNU_SOURCE (say) doesn't mean that a program that loads that library must also be built with _GNU_SOURCE. For one thing, that's hard to police. However, there are already combinations that could break, e.g., mixing -D_FILE_OFFSET_BITS=64 with -D_FILE_OFFSET_BITS=32 would be broken if this define changes off_t. So, maybe having _SC_MINSIGSTKSZ_SOURCE break things in this way is an acceptable compromise. Interfaces that depend on the value of MINSIGSTKSZ or SIGSTKSZ are possible, but probably rare in practice -- I don't know of a specific example. Cheers ---Dave
