Re: [PATCH] modpost: remove use of non-standard strsep() in HOSTCC code

From: Masahiro Yamada
Date: Sun Jun 28 2020 - 01:55:58 EST


On Thu, Jun 25, 2020 at 5:47 PM H. Nikolaus Schaller <hns@xxxxxxxxxxxxx> wrote:
>
> strsep() is neither standard C nor POSIX and used outside
> the kernel code here. Using it here requires that the
> build host supports it out of the box which is e.g.
> not true for a Darwin build host and using a cross-compiler.
> This leads to:
>
> scripts/mod/modpost.c:145:2: warning: implicit declaration of function 'strsep' [-Wimplicit-function-declaration]
> return strsep(stringp, "\n");
> ^
>
> and a segfault when running MODPOST.
>
> See also: https://stackoverflow.com/a/7219504
>
> So let's add some lines of code separating the string at the
> next newline character instead of using strsep(). It does not
> hurt kernel size or speed since this code is run on the build host.
>
> Fixes: ac5100f5432967 ("modpost: add read_text_file() and get_line() helpers")
> Signed-off-by: H. Nikolaus Schaller <hns@xxxxxxxxxxxxx>
> ---
> scripts/mod/modpost.c | 7 ++++++-
> 1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
> index 6aea65c65745..8fe63989c6e1 100644
> --- a/scripts/mod/modpost.c
> +++ b/scripts/mod/modpost.c
> @@ -138,11 +138,16 @@ char *read_text_file(const char *filename)
>
> char *get_line(char **stringp)
> {
> + char *p;
> /* do not return the unwanted extra line at EOF */
> if (*stringp && **stringp == '\0')

This check does not make sense anymore.

Previously, get_line(NULL) returns NULL.

With your patch, get_line(NULL) crashes
due to NULL-pointer dereference.



> return NULL;
>
> - return strsep(stringp, "\n");
> + p = *stringp;
> + while (**stringp != '\n')
> + (*stringp)++;


Is this a safe conversion?

If the input file does not contain '\n' at all,
this while-loop continues running,
and results in the segmentation fault
due to buffer over-run.



> + *(*stringp)++ = '\0';
> + return p;
> }



How about this?

char *get_line(char **stringp)
{
char *orig = *stringp;
char *next;

/* do not return the unwanted extra line at EOF */
if (!orig || *orig == '\0')
return NULL;

next = strchr(orig, '\n');
if (next)
*next++ = '\0';

*stringp = next;

return orig;
}




> /* A list of all modules we processed */
> --
> 2.26.2
>


--
Best Regards
Masahiro Yamada