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

[Efraim Flashner <efraim@flashner.co.il>]

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On Tue, Feb 14, 2017 at 09:47:30AM +0100, Ludovic Courtès wrote:
> Efraim Flashner <efraim@flashner.co.il> skribis:
> 
> > man2 clone: EINVAL: ... on aarch64, child_stack must be a multiple of 16.
> >
> > * nix/libstore/build.cc (DerivationGoal::startBuilder): When on aarch64,
> > when calling clone(), increment the stack by 16.
> > ---
> >  nix/libstore/build.cc | 7 ++++++-
> >  1 file changed, 6 insertions(+), 1 deletion(-)
> >
> > diff --git a/nix/libstore/build.cc b/nix/libstore/build.cc
> > index cebc404d1..362b2d91d 100644
> > --- a/nix/libstore/build.cc
> > +++ b/nix/libstore/build.cc
> > @@ -2008,7 +2008,12 @@ 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);
> > +// if statements are hard, fix this
> > +//#if __AARCH64__
> > +	pid = clone(childEntry, stack + sizeof(stack) - 16, flags, this);
> > +//#else
> > +//	pid = clone(childEntry, stack + sizeof(stack) - 8, flags, this);
> > +//#endif
> 
> I think we can make it unconditional.  Could you test whether the
> attached patch works for aarch64?
> 
> Thanks!
> 
> Ludo’.
> 

I don't get 'failed to clone' or similar error like I did without
either patch, so it looks good.

> 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


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