The point of this is to cheaply detect all stack overflows using a guard page. For a guard page to actually detect all stack overflows, we need to ensure that the code touches each page of the stack in the right order, otherwise it could skip the guard page and write outside the stack. That is very bad for languages such as Rust which provides memory safety, so it currently does an explicit comparison against the end of the stack for each function, which is again bad for performance. This would correspond to GCC's -fstack-check (if that worked). On Thu, Jul 31, 2014 at 6:40 PM, Philip Reames <listmail at philipreames.com> wrote:> Giving a bit of background and motivation would be good here. What are > you trying to accomplish and why? > > Philip > > > On 07/28/2014 04:16 PM, John Kåre Alsaker wrote: > > Hi, I want to add a stack probe function attribute which would insert > stack probes on all platforms, not just Windows. This will be useful for > Rust since it must guarantee that the stack can't overflow, which it > currently abuses the segmented stack support for. I'm not sure which kind > of attribute is appropriate here. It must be added to the caller when > inlined and clients of LLVM should be able to tell if code generation > supports it. I would like some tips on how to implement this. > > > _______________________________________________ > LLVM Developers mailing listLLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.eduhttp://lists.cs.uiuc.edu/mailman/listinfo/llvmdev > > >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20140801/1314294d/attachment.html>
Thanks for the explanation. I'm used to hearing the term "stack banging" used for this mechanism, but I understand your objective. I believe having a general mechanism here would be valuable, but only if the implementation doesn't make assumptions about runtime environment. For example, let's say my runtime uses a three page guard region and considers anything in that region to be a stack expansion. This should work with your attribute. There's also different ways of implementing this. Depending on your runtime, you might want to a) call a function, b) emit some special loads. There's also numerous optimizations which apply for the later implementation choice. Worth noting is that stack banging only when the stack size is larger than a page size is NOT sufficient unless you can *prove* that every smaller frame actually reads or writes to the frame before calling a subroutine. As an example, consider the following recursive function: int test(int i) { char buff[50]; if( i == 0 ) return 0; else return test(i-1); } The arguments will be passed in registers. The buffer won't be initialized (assuming it's not compiled away), and you'll push a stack frame without touching the stack memory. It would be nice if your attribute could also represent an explicit conditional check implementation as well. Also, are you expecting the runtime to be able to throw an exception at the site of the check? If so, there's a bunch of other issues which need handled. Straw man ideas: - Start with an string attribute, work out the semantics and implementation, then propose a "real attribute" once we've settled on a workable implementation. - Pick a more generic name. Possibly "StackOverflowGuard"? - Use two parameters. First, "minimum guard region size" (non-negative integer number of bytes). Second, "test mechanism" (enum (FuncCall, Load, Store, ConditionalCheck)). For the conditional check version, you'd need a way to specify a failure handler. For the func call version, you need a way to set the routine. Now, I realize this is well beyond what you originally wanted to implement. If you wanted to make the minimum change you could to support forcing the enable of the existing stack probe mechanism, we can discuss specifically that. I'd lean away from a general attribute for that purpose, but am open to being convinced otherwise. :) Philip On 07/31/2014 07:27 PM, John Kåre Alsaker wrote:> The point of this is to cheaply detect all stack overflows using a > guard page. For a guard page to actually detect all stack overflows, > we need to ensure that the code touches each page of the stack in the > right order, otherwise it could skip the guard page and write outside > the stack. That is very bad for languages such as Rust which provides > memory safety, so it currently does an explicit comparison against the > end of the stack for each function, which is again bad for > performance. This would correspond to GCC's -fstack-check (if that > worked). > > > On Thu, Jul 31, 2014 at 6:40 PM, Philip Reames > <listmail at philipreames.com <mailto:listmail at philipreames.com>> wrote: > > Giving a bit of background and motivation would be good here. > What are you trying to accomplish and why? > > Philip > > > On 07/28/2014 04:16 PM, John Kåre Alsaker wrote: >> Hi, I want to add a stack probe function attribute which would >> insert stack probes on all platforms, not just Windows. This will >> be useful for Rust since it must guarantee that the stack can't >> overflow, which it currently abuses the segmented stack support >> for. I'm not sure which kind of attribute is appropriate here. It >> must be added to the caller when inlined and clients of LLVM >> should be able to tell if code generation supports it. I would >> like some tips on how to implement this. >> >> >> _______________________________________________ >> LLVM Developers mailing list >> LLVMdev at cs.uiuc.edu <mailto:LLVMdev at cs.uiuc.edu> http://llvm.cs.uiuc.edu >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev > >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20140801/17717fb6/attachment.html>
On Fri, Aug 1, 2014 at 11:12 PM, Philip Reames <listmail at philipreames.com> wrote:> Thanks for the explanation. I'm used to hearing the term "stack banging" > used for this mechanism, but I understand your objective. > > I believe having a general mechanism here would be valuable, but only if > the implementation doesn't make assumptions about runtime environment. For > example, let's say my runtime uses a three page guard region and considers > anything in that region to be a stack expansion. This should work with > your attribute. > > There's also different ways of implementing this. Depending on your > runtime, you might want to a) call a function, b) emit some special loads. > There's also numerous optimizations which apply for the later > implementation choice. >> Worth noting is that stack banging only when the stack size is larger than > a page size is NOT sufficient unless you can *prove* that every smaller > frame actually reads or writes to the frame before calling a subroutine. > > As an example, consider the following recursive function: > int test(int i) { char buff[50]; if( i == 0 ) return 0; else return > test(i-1); } > > The arguments will be passed in registers. The buffer won't be > initialized (assuming it's not compiled away), and you'll push a stack > frame without touching the stack memory. >I don't think that will happen unless there's a separate call stack. Either the call instruction itself will write to the stack or it will be tail recursive call and the stack will be reused.> > > It would be nice if your attribute could also represent an explicit > conditional check implementation as well. >The "split-stack" attribute can be used for that, and that's what Rust currently does.> > > Also, are you expecting the runtime to be able to throw an exception at > the site of the check? If so, there's a bunch of other issues which need > handled. > > > Straw man ideas: > - Start with an string attribute, work out the semantics and > implementation, then propose a "real attribute" once we've settled on a > workable implementation. >I seemed to have read that string attributes were for platform specific things, would that be incorrect?> - Pick a more generic name. Possibly "StackOverflowGuard"? > - Use two parameters. First, "minimum guard region size" (non-negative > integer number of bytes). Second, "test mechanism" (enum (FuncCall, Load, > Store, ConditionalCheck)). For the conditional check version, you'd need a > way to specify a failure handler. For the func call version, you need a > way to set the routine. > > > Now, I realize this is well beyond what you originally wanted to > implement. If you wanted to make the minimum change you could to support > forcing the enable of the existing stack probe mechanism, we can discuss > specifically that. I'd lean away from a general attribute for that > purpose, but am open to being convinced otherwise. :) >I think this should be limited to only lightweight stack probing, which either emits probe instructions or calls a function (provided by libgcc/compiler-rt) which does exactly the same. One extension I'm open for is to always force a probe which ensures the stack overflow happens in a well defined point in the prologue. That could be used for languages which can recover from stack overflows.> > Philip > > > > On 07/31/2014 07:27 PM, John Kåre Alsaker wrote: > > The point of this is to cheaply detect all stack overflows using a guard > page. For a guard page to actually detect all stack overflows, we need to > ensure that the code touches each page of the stack in the right order, > otherwise it could skip the guard page and write outside the stack. That is > very bad for languages such as Rust which provides memory safety, so it > currently does an explicit comparison against the end of the stack for each > function, which is again bad for performance. This would correspond to > GCC's -fstack-check (if that worked). > > > On Thu, Jul 31, 2014 at 6:40 PM, Philip Reames <listmail at philipreames.com> > wrote: > >> Giving a bit of background and motivation would be good here. What are >> you trying to accomplish and why? >> >> Philip >> >> >> On 07/28/2014 04:16 PM, John Kåre Alsaker wrote: >> >> Hi, I want to add a stack probe function attribute which would insert >> stack probes on all platforms, not just Windows. This will be useful for >> Rust since it must guarantee that the stack can't overflow, which it >> currently abuses the segmented stack support for. I'm not sure which kind >> of attribute is appropriate here. It must be added to the caller when >> inlined and clients of LLVM should be able to tell if code generation >> supports it. I would like some tips on how to implement this. >> >> >> _______________________________________________ >> LLVM Developers mailing listLLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.eduhttp://lists.cs.uiuc.edu/mailman/listinfo/llvmdev >> >> >> > >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20140802/7978d906/attachment.html>