Re: [PATCH] readkey: shrink table of known escape sequences in size

[Sascha Hauer <s.hauer@pengutronix.de>]

On Mon, Sep 28, 2020 at 02:39:42PM +0200, Ahmad Fatoum wrote:
> Instead of storing pointers to 4-byte strings, we could just store the
> characters directly in the struct. Can save us up to 18 pointers worth
> of space.
> 
> We could save 18 x 1 byte more of static storage by making the strings
> 3 bytes of length as the nul byte need not be stored explicitly for
> strings of maximal length.
> 
> For clarity, this optimization is omitted for now. We still use strncmp,
> instead of strcmp use runs risk of accidental out of bounds reads when
> longer escape sequences are added. This can't occur with strncmp.
> 
> Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
> ---
> v1 -> v2:
>   - Maintain a nul delimiter always at the cost of 18 more bytes
>     (Sascha)
> ---
>  lib/readkey.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/lib/readkey.c b/lib/readkey.c
> index c26e9d51aba9..6a4ddf14a99d 100644
> --- a/lib/readkey.c
> +++ b/lib/readkey.c
> @@ -20,8 +20,10 @@
>  #include <linux/ctype.h>
>  #include <readkey.h>
>  
> +#define MAX_ESC_LEN 4
> +
>  struct esc_cmds {
> -	const char *seq;
> +	const char seq[MAX_ESC_LEN];
>  	unsigned char val;
>  };
>  
> @@ -49,7 +51,7 @@ static const struct esc_cmds esccmds[] = {
>  int read_key(void)
>  {
>  	unsigned char c;
> -	unsigned char esc[5];
> +	unsigned char esc[MAX_ESC_LEN + 2];
>  	c = getchar();
>  
>  	if (c == 27) {
> @@ -67,7 +69,7 @@ int read_key(void)
>  		}
>  		esc[i] = 0;
>  		for (i = 0; i < ARRAY_SIZE(esccmds); i++){
> -			if (!strcmp(esc, esccmds[i].seq))
> +			if (!strncmp(esc, esccmds[i].seq, MAX_ESC_LEN))
>  				return esccmds[i].val;

strncmp() gives you undesired results when 'esc' is longer than
MAX_ESC_LEN. Please don't start proving me that this can't happen in the
current code. I still don't think that saving a few bytes is worth the
hassle.

Sascha


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