Hi Thomas,

On Tue, Mar 7, 2023 at 9:23 AM Thomas Zimmermann [off-list ref] wrote:

Am 07.03.23 um 08:53 schrieb Geert Uytterhoeven:

quoted

On Mon, Mar 6, 2023 at 5:00 PM Thomas Zimmermann [off-list ref] wrote:

quoted

Introduce struct option_iter and helpers to parse command-line
options with comma-separated key-value pairs. Then convert fbdev
drivers to the new interface. Fixes a memory leak in the parsing of
the video= option.

Before commit 73ce73c30ba9 ("fbdev: Transfer video= option strings to
caller; clarify ownership"), a call to fb_get_options() either
returned an internal string or a duplicated string; hence ownership of
the string's memory buffer was not well defined, but depended on how
users specified the video= option on the kernel command line. For
global settings, the caller owned the returned memory and for per-driver
settings, fb_get_options() owned the memory. As calling drivers were
unable to detect the case, the memory was leaked.

Commit 73ce73c30ba9 ("fbdev: Transfer video= option strings to caller;
clarify ownership") changed sematics to caller-owned strings. Drivers
still leaked the memory, but at least ownership was clear.

This patchset fixes the memory leak and changes string ownership back
to fb_get_options(). Patch 1 introduces struct option_iter and a few
helpers. The interface takes an option string, such as video=, in the
common form value1,key2:value2,value3 etc and returns the individial
comma-separated pairs. Various modules use this pattern, so the code
is located under lib/.

Patches 2 to 98 go through fbdev drivers and convert them to the new
interface. This often requires a number of cleanups. A driver would
typically refer to the option string's video mode. Such strings are now
copied to driver-allocated memory so that drivers don't refer directly
to the option string's memory. The option iterator then replaces manual
parsing loops based on strsep(",").

Thanks for your series!

Unfortunately I cannot say I'm thrilled about this: you are replacing
a single small dynamic memory leak by 36 larger static memory leaks.

That's fair enough.

quoted

Am I missing something?

The current size of the videomode buffers is ridiculously large. I just
needed something that could hold the string. A long mode description
might look like

   1920x1080MR-32@120ime

which has 21 characters. 32-byte buffers would probably be more than enough.

But there are a few exceptions...

I think it should also be possible to do a simple kstrdup() on the given
videomode string and free the copy in the module's _fini function.

That sounds like the sanest approach to me.

That also brings up the question of these MODULE ifdefs. Almost all of
the fbdev drivers only parse the command-line option if they are not
build as a module. Do you know why? Because of the awkward semantics of
the old fb_get_options()?

That's just historical: to get to see anything on the console (on
the graphics hardware without VGA text mode that fbdev was originally
developed for), you needed to have your main fbdev driver builtin.
Drivers for secondary displays could be loadable modules, and using
fbset for those offered more flexibility than a module parameter.

I think this should be changed so that they
always respect the video= parameter.

I agree that makes sense.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds