Re: [PATCH v3 1/2] firmware: arm_ffa: Fix Endpoint Memory Access Descriptor offset calculation

[Mostafa Saleh <smostafa@google.com>]

On Tue, May 12, 2026 at 12:44:41PM +0000, Sebastian Ene wrote:
> Use the descriptor's `ep_mem_offset` to calculate the start of the endpoint
> memory access array and to comply with the FF-A spec instead of defaulting
> to `sizeof(struct ffa_mem_region)`.
> This requires moving `ffa_mem_region_additional_setup()` earlier in the setup
> flow.
> Also, add sanity checks to ensure the calculated descriptor offsets do not
> exceed `max_fragsize`.
> 
> Signed-off-by: Sebastian Ene <sebastianene@google.com>
> ---
>  drivers/firmware/arm_ffa/driver.c | 14 ++++++++++----
>  include/linux/arm_ffa.h           |  2 +-
>  2 files changed, 11 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/firmware/arm_ffa/driver.c b/drivers/firmware/arm_ffa/driver.c
> index eb2782848283..56b166290b24 100644
> --- a/drivers/firmware/arm_ffa/driver.c
> +++ b/drivers/firmware/arm_ffa/driver.c
> @@ -685,18 +685,25 @@ ffa_setup_and_transmit(u32 func_id, void *buffer, u32 max_fragsize,
>  	struct ffa_composite_mem_region *composite;
>  	struct ffa_mem_region_addr_range *constituents;
>  	struct ffa_mem_region_attributes *ep_mem_access;
> -	u32 idx, frag_len, length, buf_sz = 0, num_entries = sg_nents(args->sg);
> +	u32 idx, frag_len, length, buf_sz = 0, num_entries = sg_nents(args->sg), ep_offset;
>  
>  	mem_region->tag = args->tag;
>  	mem_region->flags = args->flags;
>  	mem_region->sender_id = drv_info->vm_id;
>  	mem_region->attributes = ffa_memory_attributes_get(func_id);
> +
> +	ffa_mem_region_additional_setup(drv_info->version, mem_region);
>  	composite_offset = ffa_mem_desc_offset(buffer, args->nattrs,
>  					       drv_info->version);
> +	if (composite_offset > max_fragsize - sizeof(struct ffa_composite_mem_region))
> +		return -ENXIO;

nit: This driver seems to use sizeof() with variable name rather than
type (except for one place) so it may be good to keep that.

>  
>  	for (idx = 0; idx < args->nattrs; idx++) {
> -		ep_mem_access = buffer +
> -			ffa_mem_desc_offset(buffer, idx, drv_info->version);
> +		ep_offset = ffa_mem_desc_offset(buffer, idx, drv_info->version);
> +		if (ep_offset > max_fragsize - sizeof(struct ffa_mem_region_attributes))
> +			return -ENXIO;
> +
> +		ep_mem_access = buffer + ep_offset;
>  		ep_mem_access->receiver = args->attrs[idx].receiver;
>  		ep_mem_access->attrs = args->attrs[idx].attrs;
>  		ep_mem_access->composite_off = composite_offset;
> @@ -708,7 +715,6 @@ ffa_setup_and_transmit(u32 func_id, void *buffer, u32 max_fragsize,
>  	}
>  	mem_region->handle = 0;
>  	mem_region->ep_count = args->nattrs;
> -	ffa_mem_region_additional_setup(drv_info->version, mem_region);
>  
>  	composite = buffer + composite_offset;
>  	composite->total_pg_cnt = ffa_get_num_pages_sg(args->sg);
> diff --git a/include/linux/arm_ffa.h b/include/linux/arm_ffa.h
> index 81e603839c4a..62d67dae8b70 100644
> --- a/include/linux/arm_ffa.h
> +++ b/include/linux/arm_ffa.h
> @@ -445,7 +445,7 @@ ffa_mem_desc_offset(struct ffa_mem_region *buf, int count, u32 ffa_version)
>  	if (!FFA_MEM_REGION_HAS_EP_MEM_OFFSET(ffa_version))
>  		offset += offsetof(struct ffa_mem_region, ep_mem_offset);
>  	else
> -		offset += sizeof(struct ffa_mem_region);
> +		offset += buf->ep_mem_offset;

Does it make sense to also set buf->ep_mem_offset for the other
case in ffa_mem_region_additional_setup() and then add this
unconditionally here?

Thanks,
Mostafa


>  
>  	return offset;
>  }
> -- 
> 2.54.0.563.g4f69b47b94-goog
>