linux-kernel - Re: [PATCH v3 1/1] pgo: Fix allocate

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Date:   Fri, 4 Jun 2021 10:04:40 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Jarmo Tiitto <jarmo.tiitto@...il.com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Bill Wendling <wcw@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux@...glegroups.com, linux-kernel@...r.kernel.org
Cc:     morbo@...gle.com
Subject: Re: [PATCH v3 1/1] pgo: Fix allocate_node() v2

On 6/4/2021 9:58 AM, Jarmo Tiitto wrote:
> When clang instrumentation eventually calls allocate_node()
> the struct llvm_prf_data *p argument tells us from what section
> we should reserve the vnode: It either points into vmlinux's
> core __llvm_prf_data section or some loaded module's
> __llvm_prf_data section.
> 
> But since we don't have access to corresponding
> __llvm_prf_vnds section(s) for any module, the function
> should return just NULL and ignore any profiling attempts
> from modules for now.
> 
> Signed-off-by: Jarmo Tiitto <jarmo.tiitto@...il.com>

Reviewed-by: Nathan Chancellor <nathan@...nel.org>

> ---
> Based on Kees and others feedback here is v3 patch
> that clarifies why the current checks in allocate_node()
> are flawed. I did fair amount of KGDB time on it.
> 
> The commit is based on kees/for-next/clang/features tree,
> hopefully this is ok. Should I have based it on linux-next
> instead?
> 
> I grep -R'd where the memory_contains() can be found and it is only
> found in #include <asm-generic/sections.h>
> 
> I cross my fingers and await if this is my first accepted patch. :-)
> ---
>   kernel/pgo/instrument.c | 23 ++++++++++++++---------
>   1 file changed, 14 insertions(+), 9 deletions(-)
> 
> diff --git a/kernel/pgo/instrument.c b/kernel/pgo/instrument.c
> index 0e07ee1b17d9..c69b4f7ebaad 100644
> --- a/kernel/pgo/instrument.c
> +++ b/kernel/pgo/instrument.c
> @@ -18,6 +18,7 @@
>   
>   #define pr_fmt(fmt)	"pgo: " fmt
>   
> +#include <asm-generic/sections.h>
>   #include <linux/bitops.h>
>   #include <linux/kernel.h>
>   #include <linux/export.h>
> @@ -55,17 +56,21 @@ void prf_unlock(unsigned long flags)
>   static struct llvm_prf_value_node *allocate_node(struct llvm_prf_data *p,
>   						 u32 index, u64 value)
>   {
> -	if (&__llvm_prf_vnds_start[current_node + 1] >= __llvm_prf_vnds_end)
> -		return NULL; /* Out of nodes */
> -
> -	current_node++;
> -
> -	/* Make sure the node is entirely within the section */
> -	if (&__llvm_prf_vnds_start[current_node] >= __llvm_prf_vnds_end ||
> -	    &__llvm_prf_vnds_start[current_node + 1] > __llvm_prf_vnds_end)
> +	const int max_vnds = prf_vnds_count();
> +
> +	/*
> +	 * Check that p is within vmlinux __llvm_prf_data section.
> +	 * If not, don't allocate since we can't handle modules yet.
> +	 */
> +	if (!memory_contains(__llvm_prf_data_start,
> +		__llvm_prf_data_end, p, sizeof(*p)))
>   		return NULL;
>   
> -	return &__llvm_prf_vnds_start[current_node];
> +	if (WARN_ON_ONCE(current_node >= max_vnds))
> +		return NULL; /* Out of nodes */
> +
> +	/* reserve vnode for vmlinux */
> +	return &__llvm_prf_vnds_start[current_node++];
>   }
>   
>   /*
> 
> base-commit: 5d0cda65918279ada060417c5fecb7e86ccb3def
>