[PATCH] tracefs utils: Do not free a the buffer on a zero size str_read_file()

[PATCH] tracefs utils: Do not free a the buffer on a zero size str_read_file()

[Steven Rostedt]

From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>

If a file has no size (nothing is read), then str_read_file() frees the
buffer and returns zero. The problem is that all callers of str_read_file()
uses the buffer supplied if the value returned is not a negative. This
causes the freed buffer being used by the callers if the file read existed
but had no content.

This is apparent when using a copy of the tracefs directory, where some file
exist, but have no content, then loading the events would cause a segfault.

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
 tracefs-utils.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tracefs-utils.c b/tracefs-utils.c
index 326b455..690506c 100644
--- a/tracefs-utils.c
+++ b/tracefs-utils.c
@@ -220,7 +220,7 @@ __hidden int str_read_file(const char *file, char **buffer)
 	} while (r > 0);
 
 	close(fd);
-	if (r == 0 && size > 0) {
+	if (r == 0) {
 		buf[size] = '\0';
 		*buffer = buf;
 	} else
-- 
2.25.4

Re: [PATCH] tracefs utils: Do not free a the buffer on a zero size str_read_file()

[Tzvetomir Stoyanov]

On Mon, Nov 23, 2020 at 10:35 PM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
>
> If a file has no size (nothing is read), then str_read_file() frees the
> buffer and returns zero. The problem is that all callers of str_read_file()
> uses the buffer supplied if the value returned is not a negative. This
> causes the freed buffer being used by the callers if the file read existed
> but had no content.
>
> This is apparent when using a copy of the tracefs directory, where some file
> exist, but have no content, then loading the events would cause a segfault.
>
> Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> ---
>  tracefs-utils.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/tracefs-utils.c b/tracefs-utils.c
> index 326b455..690506c 100644
> --- a/tracefs-utils.c
> +++ b/tracefs-utils.c
> @@ -220,7 +220,7 @@ __hidden int str_read_file(const char *file, char **buffer)
>         } while (r > 0);
>
>         close(fd);
> -       if (r == 0 && size > 0) {
> +       if (r == 0) {
>                 buf[size] = '\0';

If size is 0, in case of an empty file, then buf should also be NULL
and this assignment
will be on invalid memory.

>                 *buffer = buf;
>         } else
> --
> 2.25.4
>


-- 
Tzvetomir (Ceco) Stoyanov
VMware Open Source Technology Center

Re: [PATCH] tracefs utils: Do not free a the buffer on a zero size str_read_file()

[Steven Rostedt]

On Tue, 24 Nov 2020 07:44:14 +0200
Tzvetomir Stoyanov <tz.stoyanov@gmail.com> wrote:

> > index 326b455..690506c 100644
> > --- a/tracefs-utils.c
> > +++ b/tracefs-utils.c
> > @@ -220,7 +220,7 @@ __hidden int str_read_file(const char *file, char **buffer)
> >         } while (r > 0);
> >
> >         close(fd);
> > -       if (r == 0 && size > 0) {
> > +       if (r == 0) {
> >                 buf[size] = '\0';  
> 
> If size is 0, in case of an empty file, then buf should also be NULL
> and this assignment
> will be on invalid memory.

I quickly realized that, and sent out a v2 ;-)

 https://lore.kernel.org/r/20201123154607.5e43e1ff@gandalf.local.home

-- Steve

> 
> >                 *buffer = buf;
> >         } else
> > --