[PATCH] D154911: Enabling fstack_clash_protection for arm32 bit, thumb and thumb2 mode

[Tamar Christina via Phabricator via llvm-commits]

tnfchris added inline comments.


================
Comment at: clang/lib/Driver/ToolChains/Clang.cpp:3460
+  } else if (EffectiveTriple.isArm() || EffectiveTriple.isThumb()) {
+    CmdArgs.push_back("-mstack-probe-size=1024");
+  }
----------------
efriedma wrote:
> tnfchris wrote:
> > efriedma wrote:
> > > Why 1024?
> > 1024 was experimentally determined by Arm and is part of the ABI for stack clash (which has not yet been published).  It was determined by examining a large number of programs and looking at the function stack usages.  1024 covers 80-90% of programs such that we can minimize the number of probes required in the average cases. 
> There are actually multiple numbers involved here, no?  One is the spacing of probes, i.e. if allocating a large amount of stack, how many times you need to probe; this is basically the page size of the target. the other is how much unprobed space a function is allowed to allocate before calling another function. Referring to the the AArch64 patch, -mstack-probe-size is the former, the hardcoded "1024" is the latter.
I hadn't looked at the patch in detail yet, I thought this was the probing offset.  But you're right, what I thought of was `StackClashCallerGuard`,  if `stack-probe-size` indeed the guard size itself, then yeah this would be wrong.  It seems incorrect to allow it smaller than the page size.


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CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D154911/new/

https://reviews.llvm.org/D154911