Re: [PATCH v3 34/35] codetag: debug: introduce OBJEXTS_ALLOC_FAIL to mark failed slab_ext allocations

[Kees Cook]

On Mon, Feb 12, 2024 at 01:39:20PM -0800, Suren Baghdasaryan wrote:
> If slabobj_ext vector allocation for a slab object fails and later on it
> succeeds for another object in the same slab, the slabobj_ext for the
> original object will be NULL and will be flagged in case when
> CONFIG_MEM_ALLOC_PROFILING_DEBUG is enabled.
> Mark failed slabobj_ext vector allocations using a new objext_flags flag
> stored in the lower bits of slab->obj_exts. When new allocation succeeds
> it marks all tag references in the same slabobj_ext vector as empty to
> avoid warnings implemented by CONFIG_MEM_ALLOC_PROFILING_DEBUG checks.
> 
> Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> ---
>  include/linux/memcontrol.h |  4 +++-
>  mm/slab.h                  | 25 +++++++++++++++++++++++++
>  mm/slab_common.c           | 22 +++++++++++++++-------
>  3 files changed, 43 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 2b010316016c..f95241ca9052 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -365,8 +365,10 @@ enum page_memcg_data_flags {
>  #endif /* CONFIG_MEMCG */
>  
>  enum objext_flags {
> +	/* slabobj_ext vector failed to allocate */
> +	OBJEXTS_ALLOC_FAIL = __FIRST_OBJEXT_FLAG,
>  	/* the next bit after the last actual flag */
> -	__NR_OBJEXTS_FLAGS  = __FIRST_OBJEXT_FLAG,
> +	__NR_OBJEXTS_FLAGS  = (__FIRST_OBJEXT_FLAG << 1),
>  };
>  
>  #define OBJEXTS_FLAGS_MASK (__NR_OBJEXTS_FLAGS - 1)
> diff --git a/mm/slab.h b/mm/slab.h
> index cf332a839bf4..7bb3900f83ef 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -586,9 +586,34 @@ static inline void mark_objexts_empty(struct slabobj_ext *obj_exts)
>  	}
>  }
>  
> +static inline void mark_failed_objexts_alloc(struct slab *slab)
> +{
> +	slab->obj_exts = OBJEXTS_ALLOC_FAIL;

Uh, does this mean slab->obj_exts is suddenly non-NULL? Is everything
that accesses obj_exts expecting this?

-Kees

> +}
> +
> +static inline void handle_failed_objexts_alloc(unsigned long obj_exts,
> +			struct slabobj_ext *vec, unsigned int objects)
> +{
> +	/*
> +	 * If vector previously failed to allocate then we have live
> +	 * objects with no tag reference. Mark all references in this
> +	 * vector as empty to avoid warnings later on.
> +	 */
> +	if (obj_exts & OBJEXTS_ALLOC_FAIL) {
> +		unsigned int i;
> +
> +		for (i = 0; i < objects; i++)
> +			set_codetag_empty(&vec[i].ref);
> +	}
> +}
> +
> +
>  #else /* CONFIG_MEM_ALLOC_PROFILING_DEBUG */
>  
>  static inline void mark_objexts_empty(struct slabobj_ext *obj_exts) {}
> +static inline void mark_failed_objexts_alloc(struct slab *slab) {}
> +static inline void handle_failed_objexts_alloc(unsigned long obj_exts,
> +			struct slabobj_ext *vec, unsigned int objects) {}
>  
>  #endif /* CONFIG_MEM_ALLOC_PROFILING_DEBUG */
>  
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index d5f75d04ced2..489c7a8ba8f1 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -214,29 +214,37 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
>  			gfp_t gfp, bool new_slab)
>  {
>  	unsigned int objects = objs_per_slab(s, slab);
> -	unsigned long obj_exts;
> -	void *vec;
> +	unsigned long new_exts;
> +	unsigned long old_exts;
> +	struct slabobj_ext *vec;
>  
>  	gfp &= ~OBJCGS_CLEAR_MASK;
>  	/* Prevent recursive extension vector allocation */
>  	gfp |= __GFP_NO_OBJ_EXT;
>  	vec = kcalloc_node(objects, sizeof(struct slabobj_ext), gfp,
>  			   slab_nid(slab));
> -	if (!vec)
> +	if (!vec) {
> +		/* Mark vectors which failed to allocate */
> +		if (new_slab)
> +			mark_failed_objexts_alloc(slab);
> +
>  		return -ENOMEM;
> +	}
>  
> -	obj_exts = (unsigned long)vec;
> +	new_exts = (unsigned long)vec;
>  #ifdef CONFIG_MEMCG
> -	obj_exts |= MEMCG_DATA_OBJEXTS;
> +	new_exts |= MEMCG_DATA_OBJEXTS;
>  #endif
> +	old_exts = slab->obj_exts;
> +	handle_failed_objexts_alloc(old_exts, vec, objects);
>  	if (new_slab) {
>  		/*
>  		 * If the slab is brand new and nobody can yet access its
>  		 * obj_exts, no synchronization is required and obj_exts can
>  		 * be simply assigned.
>  		 */
> -		slab->obj_exts = obj_exts;
> -	} else if (cmpxchg(&slab->obj_exts, 0, obj_exts)) {
> +		slab->obj_exts = new_exts;
> +	} else if (cmpxchg(&slab->obj_exts, old_exts, new_exts) != old_exts) {
>  		/*
>  		 * If the slab is already in use, somebody can allocate and
>  		 * assign slabobj_exts in parallel. In this case the existing
> -- 
> 2.43.0.687.g38aa6559b0-goog
> 

-- 
Kees Cook