linux-kernel - Re: [PATCH v2 6/9] perf tests: Use scandirat for shell script finding

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Date: Mon, 12 Feb 2024 08:06:43 -0800
From: Ian Rogers <irogers@...gle.com>
To: Namhyung Kim <namhyung@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
	Arnaldo Carvalho de Melo <acme@...nel.org>, Mark Rutland <mark.rutland@....com>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>, 
	Adrian Hunter <adrian.hunter@...el.com>, Nathan Chancellor <nathan@...nel.org>, 
	Nick Desaulniers <ndesaulniers@...gle.com>, Tom Rix <trix@...hat.com>, 
	Ravi Bangoria <ravi.bangoria@....com>, James Clark <james.clark@....com>, 
	Kan Liang <kan.liang@...ux.intel.com>, John Garry <john.g.garry@...cle.com>, 
	linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org, 
	llvm@...ts.linux.dev
Subject: Re: [PATCH v2 6/9] perf tests: Use scandirat for shell script finding

On Fri, Feb 9, 2024 at 8:41 PM Namhyung Kim <namhyung@...nel.org> wrote:
>
> On Wed, Jan 31, 2024 at 4:15 PM Ian Rogers <irogers@...gle.com> wrote:
> >
> > Avoid filename appending buffers by using openat, faccessat and
> > scandirat more widely. Turn the script's path back to a file name
> > using readlink from /proc/<pid>/fd/<fd>.
> >
> > Read the script's description using api/io.h to avoid fdopen
> > conversions. Whilst reading perform additional sanity checks on the
> > script's contents.
> >
> > Signed-off-by: Ian Rogers <irogers@...gle.com>
> > ---
> [SNIP]
> > -static const char *shell_test__description(char *description, size_t size,
> > -                                           const char *path, const char *name)
> > +static char *shell_test__description(int dir_fd, const char *name)
> >  {
> > -       FILE *fp;
> > -       char filename[PATH_MAX];
> > -       int ch;
> > +       struct io io;
> > +       char buf[128], desc[256];
> > +       int ch, pos = 0;
> >
> > -       path__join(filename, sizeof(filename), path, name);
> > -       fp = fopen(filename, "r");
> > -       if (!fp)
> > +       io__init(&io, openat(dir_fd, name, O_RDONLY), buf, sizeof(buf));
> > +       if (io.fd < 0)
> >                 return NULL;
> >
> >         /* Skip first line - should be #!/bin/sh Shebang */
> > +       if (io__get_char(&io) != '#')
> > +               goto err_out;
> > +       if (io__get_char(&io) != '!')
> > +               goto err_out;
> >         do {
> > -               ch = fgetc(fp);
> > -       } while (ch != EOF && ch != '\n');
> > -
> > -       description = fgets(description, size, fp);
> > -       fclose(fp);
> > +               ch = io__get_char(&io);
> > +               if (ch < 0)
> > +                       goto err_out;
> > +       } while (ch != '\n');
> >
> > -       /* Assume first char on line is omment everything after that desc */
> > -       return description ? strim(description + 1) : NULL;
> > +       do {
> > +               ch = io__get_char(&io);
> > +               if (ch < 0)
> > +                       goto err_out;
> > +       } while (ch == '#' || isspace(ch));
> > +       while (ch > 0 && ch != '\n') {
> > +               desc[pos++] = ch;
> > +               if (pos >= (int)sizeof(desc) - 1)
>
> Maybe (pos == sizeof(desc) - 2) ?  I'm not sure what happens if it has a
> description longer than the buffer size.

Thanks Namhyung!

sizeof(desc) - 1 == sizeof(char[256]) - 1 == 255 , so at this point
pos can at most be 255 and there is one space after pos for a trailing
'\0'.

> > +                       break;
> > +               ch = io__get_char(&io);
> > +       }
> > +       while (pos > 0 && isspace(desc[--pos]))
> > +               ;

Here pos is moved back to at least one to 254.

> > +       desc[++pos] = '\0';
>
> Wouldn't it overflow the buffer?

At this point pos can only have a maximum value of 255 which is within
the bounds of desc.

Thanks,
Ian

> Thanks,
> Namhyung
>
>
> > +       close(io.fd);
> > +       return strdup(desc);
> > +err_out:
> > +       close(io.fd);
> > +       return NULL;
> >  }