Re: [PATCH] staging: fbtft: fix unaligned access and buffer size when startbyte is used

[David Laight]

On Thu, 25 Jun 2026 18:30:41 +0800
suryasaimadhu <suryasaimadhu369@xxxxxxxxx> wrote:

> When par->startbyte is non-zero, buf is advanced by one byte creating
> an unaligned pointer for 16-bit types (u16, __be16). Dereferencing this
> unaligned pointer can cause a kernel panic on strict-alignment
> architectures.
> 
> Fix by using put_unaligned() instead of direct pointer dereference.
> 
> Also fix incorrect buffer size calculation in fbtft_write_buf_dc() call:
>   len * (sizeof(data_type) + offset)  /* wrong: multiplies offset by len */
>   len * sizeof(data_type) + offset    /* correct: one startbyte +
>                                          len items */

That should probably be a separate patch.

> 
> Signed-off-by: suryasaimadhu <suryasaimadhu369@xxxxxxxxx>
> ---
>  drivers/staging/fbtft/fbtft-bus.c | 13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/staging/fbtft/fbtft-bus.c b/drivers/staging/fbtft/fbtft-bus.c
> index 2169f8d1d..cfcf4d7e7 100644
> --- a/drivers/staging/fbtft/fbtft-bus.c
> +++ b/drivers/staging/fbtft/fbtft-bus.c
> @@ -4,6 +4,7 @@
>  #include <linux/gpio/consumer.h>
>  #include <linux/spi/spi.h>
>  #include "fbtft.h"
> +#include <linux/unaligned.h>
>  
>  /*****************************************************************************
>   *
> @@ -40,7 +41,7 @@ void func(struct fbtft_par *par, int len, ...)                                \

I'd consider changing that to:
	func(struct fbtft_par *par, int len, u8 cmd, ...)
and probably reducing len by one.

It makes it more obvious that the first parameter is mandatory and the ... is
associated data.

	David

>  		offset = 1;                                                   \
>  	}                                                                     \
>  									      \
> -	*buf = modifier((data_type)va_arg(args, unsigned int));               \
> +	put_unaligned(modifier((data_type)va_arg(args, unsigned int)), buf);  \
>  	ret = fbtft_write_buf_dc(par, par->buf, sizeof(data_type) + offset,   \
>  				 0);                                          \
>  	if (ret < 0)							      \
> @@ -52,11 +53,13 @@ void func(struct fbtft_par *par, int len, ...)                                \
>  									      \
>  	if (len) {                                                            \
>  		i = len;                                                      \
> -		while (i--)						      \
> -			*buf++ = modifier((data_type)va_arg(args,             \
> -							    unsigned int));   \
> +		while (i--) {                                                 \
> +			put_unaligned(modifier((data_type)va_arg(args,        \
> +					       unsigned int)), buf);          \
> +			buf++;                                                \
> +		}                                                             \
>  		fbtft_write_buf_dc(par, par->buf,			      \
> -				   len * (sizeof(data_type) + offset), 1);    \
> +				   len * sizeof(data_type) + offset, 1);      \
>  	}                                                                     \
>  out:									      \
>  	va_end(args);                                                         \