Re: [PATCH 3/6] daemon: On aarch64, use increments of 16 on the stack.

[Mark H Weaver <mhw@netris.org>]

Hi Efraim,

Efraim Flashner <efraim@flashner.co.il> writes:

> On Sat, Aug 05, 2017 at 02:21:55AM -0400, Mark H Weaver wrote:
>> Reviving a very old thread...
>> 
>> ludo@gnu.org (Ludovic Courtès) writes:
>> 
>> > diff --git a/nix/libstore/build.cc b/nix/libstore/build.cc
>> > index cebc404d1..9b7bb5391 100644
>> > --- a/nix/libstore/build.cc
>> > +++ b/nix/libstore/build.cc
>> > @@ -26,6 +26,7 @@
>> >  #include <errno.h>
>> >  #include <stdio.h>
>> >  #include <cstring>
>> > +#include <stdint.h>
>> >  
>> >  #include <pwd.h>
>> >  #include <grp.h>
>> > @@ -2008,7 +2009,11 @@ void DerivationGoal::startBuilder()
>> >  	char stack[32 * 1024];
>> >  	int flags = CLONE_NEWPID | CLONE_NEWNS | CLONE_NEWIPC | CLONE_NEWUTS | SIGCHLD;
>> >  	if (!fixedOutput) flags |= CLONE_NEWNET;
>> > -	pid = clone(childEntry, stack + sizeof(stack) - 8, flags, this);
>> > +
>> > +	/* Ensure proper alignment on the stack.  On aarch64, it has to be 16
>> > +	   bytes.  */
>> > +	pid = clone(childEntry, (char *)(((uintptr_t)stack + 16) & ~0xf),
>> > +		    flags, this);
>> >  	if (pid == -1)
>> >  	    throw SysError("cloning builder process");
>> >      } else
>> 
>> This patch, applied in February, contains a serious error.  The stack
>> address passed to 'clone' is supposed to be near the end of the memory
>> block allocated for the stack, and that's how it was before this patch
>> was applied.  Since this patch was applied, it now passes an address
>> very close to the *start* of the memory block.
>> 
>> This broke the daemon on mips64el in a subtle way that was rather
>> difficult to debug.  After about six months of being too busy with other
>> things to investigate properly, I finally tracked it down to this
>> change.
>> 
>> I reverted this commit.  Let's try again to find a proper fix for this
>> issue on aarch64.
>> 
>>      Thanks,
>>        Mark
>
> How about doubling the size of the stack to [32 * 1024  * 2] and

Is there a need to double the size of the stack?  If we have no reason
to think so, I'd rather leave it alone.

> changing the clone location to 'stack + sizeof(stack) - 16', does that
> work for mips64el?

The problem with (stack + sizeof(stack) - 16) is that there's no
guarantee that 'stack' will be aligned on a 16-byte boundary.  It might
be that if we add another local variable somewhere else in this
function, or if the compiler changes, we'll need to change the 16 to a
different number to make it work.

Can you try the following patch on aarch64 and report back?

     Thanks,
       Mark

--8<---------------cut here---------------start------------->8---
diff --git a/nix/libstore/build.cc b/nix/libstore/build.cc
index 693fa70c8..c5cd4bdb2 100644
--- a/nix/libstore/build.cc
+++ b/nix/libstore/build.cc
@@ -26,6 +26,7 @@
 #include <errno.h>
 #include <stdio.h>
 #include <cstring>
+#include <stdint.h>
 
 #include <pwd.h>
 #include <grp.h>
@@ -2008,11 +2009,11 @@ void DerivationGoal::startBuilder()
 	char stack[32 * 1024];
 	int flags = CLONE_NEWPID | CLONE_NEWNS | CLONE_NEWIPC | CLONE_NEWUTS | SIGCHLD;
 	if (!fixedOutput) flags |= CLONE_NEWNET;
-#ifdef __aarch64__
-	    pid = clone(childEntry, stack + sizeof(stack) - 16, flags, this);
-#else
-	    pid = clone(childEntry, stack + sizeof(stack) - 8, flags, this);
-#endif
+	/* Ensure proper alignment on the stack.  On aarch64, it has to be 16
+	   bytes.  */
+	pid = clone(childEntry,
+		    (char *)(((uintptr_t)stack + sizeof(stack) - 8) & ~0xf),
+		    flags, this);
 	if (pid == -1)
 	    throw SysError("cloning builder process");
     } else
--8<---------------cut here---------------end--------------->8---