Re: [PATCH] media: dvb-frontends: avoid stack overflow warnings with clang

[Justin Stitt]

Hi,

On Fri, Feb 16, 2024 at 8:32 AM Arnd Bergmann <arnd@xxxxxxxxxx> wrote:
>
> From: Arnd Bergmann <arnd@xxxxxxxx>
>
> A previous patch worked around a KASAN issue in stv0367, now a similar
> problem showed up with clang:
>
> drivers/media/dvb-frontends/stv0367.c:1222:12: error: stack frame size (3624) exceeds limit (2048) in 'stv0367ter_set_frontend' [-Werror,-Wframe-larger-than]
>  1214 | static int stv0367ter_set_frontend(struct dvb_frontend *fe)
>
> Rework the stv0367_writereg() function to be simpler and mark both
> register access functions as noinline_for_stack so the temporary
> i2c_msg structures do not get duplicated on the stack when KASAN_STACK
> is enabled.
>
> Fixes: 3cd890dbe2a4 ("media: dvb-frontends: fix i2c access helpers for KASAN")
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---
>  drivers/media/dvb-frontends/stv0367.c | 34 +++++++--------------------
>  1 file changed, 8 insertions(+), 26 deletions(-)
>
> diff --git a/drivers/media/dvb-frontends/stv0367.c b/drivers/media/dvb-frontends/stv0367.c
> index 48326434488c..72540ef4e5f8 100644
> --- a/drivers/media/dvb-frontends/stv0367.c
> +++ b/drivers/media/dvb-frontends/stv0367.c
> @@ -118,50 +118,32 @@ static const s32 stv0367cab_RF_LookUp2[RF_LOOKUP_TABLE2_SIZE][RF_LOOKUP_TABLE2_S
>         }
>  };
>
> -static
> -int stv0367_writeregs(struct stv0367_state *state, u16 reg, u8 *data, int len)
> +static noinline_for_stack
> +int stv0367_writereg(struct stv0367_state *state, u16 reg, u8 data)
>  {
> -       u8 buf[MAX_XFER_SIZE];
> +       u8 buf[3] = { MSB(reg), LSB(reg), data };
>         struct i2c_msg msg = {
>                 .addr = state->config->demod_address,
>                 .flags = 0,
>                 .buf = buf,
> -               .len = len + 2
> +               .len = 3,
>         };
>         int ret;
>
> -       if (2 + len > sizeof(buf)) {
> -               printk(KERN_WARNING
> -                      "%s: i2c wr reg=%04x: len=%d is too big!\n",
> -                      KBUILD_MODNAME, reg, len);
> -               return -EINVAL;
> -       }
> -
> -
> -       buf[0] = MSB(reg);
> -       buf[1] = LSB(reg);
> -       memcpy(buf + 2, data, len);

I'm curious why a copy was made at all.

Reviewed-by: Justin Stitt <justinstitt@xxxxxxxxxx>

> -
>         if (i2cdebug)
>                 printk(KERN_DEBUG "%s: [%02x] %02x: %02x\n", __func__,
> -                       state->config->demod_address, reg, buf[2]);
> +                       state->config->demod_address, reg, data);
>
>         ret = i2c_transfer(state->i2c, &msg, 1);
>         if (ret != 1)
>                 printk(KERN_ERR "%s: i2c write error! ([%02x] %02x: %02x)\n",
> -                       __func__, state->config->demod_address, reg, buf[2]);
> +                       __func__, state->config->demod_address, reg, data);
>
>         return (ret != 1) ? -EREMOTEIO : 0;
>  }
>
> -static int stv0367_writereg(struct stv0367_state *state, u16 reg, u8 data)
> -{
> -       u8 tmp = data; /* see gcc.gnu.org/bugzilla/show_bug.cgi?id=81715 */
> -
> -       return stv0367_writeregs(state, reg, &tmp, 1);
> -}
> -
> -static u8 stv0367_readreg(struct stv0367_state *state, u16 reg)
> +static noinline_for_stack
> +u8 stv0367_readreg(struct stv0367_state *state, u16 reg)
>  {
>         u8 b0[] = { 0, 0 };
>         u8 b1[] = { 0 };
> --
> 2.39.2
>

Thanks
Justin