Re: [rfc 2/2] x86, bts: use physically non-contiguous trace buffer

[Andi Kleen]

Markus Metzger wrote:
> Use vmalloc to allocate the branch trace buffer.
>
> Peter Zijlstra suggested to use vmalloc rather than kmalloc to
> allocate the potentially multi-page branch trace buffer.
>
> Is there a way to have vmalloc allocate a physically non-contiguous
> buffer for test purposes? Ideally, the memory area would have big
> holes in it with sensitive data in between so I would know immediately
> when this is overwritten.

For test purposes you could hack vmalloc.c to allocate more pages
and only put in every second/third/... into the mapping. You would
just need to add another to loop to __vmalloc_area_node() that allocates
more. That should give you non continuous mappings unless you're really unlucky.

-Andi


> Reported-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Markus Metzger <markus.t.metzger@xxxxxxxxx>
> ---
>  arch/x86/kernel/ptrace.c |    5 	3 +	2 -	0 !
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> Index: b/arch/x86/kernel/ptrace.c
> ===================================================================
> --- a/arch/x86/kernel/ptrace.c
> +++ b/arch/x86/kernel/ptrace.c
> @@ -22,6 +22,7 @@
>  #include <linux/seccomp.h>
>  #include <linux/signal.h>
>  #include <linux/workqueue.h>
> +#include <linux/vmalloc.h>
>  
>  #include <asm/uaccess.h>
>  #include <asm/pgtable.h>
> @@ -626,7 +627,7 @@ static int alloc_bts_buffer(struct bts_c
>  	if (err < 0)
>  		return err;
>  
> -	buffer = kzalloc(size, GFP_KERNEL);
> +	buffer = vmalloc(size);
>  	if (!buffer)
>  		goto out_refund;
>  
> @@ -646,7 +647,7 @@ static inline void free_bts_buffer(struc
>  	if (!context->buffer)
>  		return;
>  
> -	kfree(context->buffer);
> +	vfree(context->buffer);
>  	context->buffer = NULL;
>  
>  	refund_locked_memory(context->mm, context->size);

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