libXfont2 - how to prevent memory leak?

[Ulrich Sibiller]

ping

On Sat, Oct 17, 2020 at 11:14 PM Ulrich Sibiller
<ulrich.sibiller at gmail.com> wrote:
>
> Hello,
>
> I have some Xserver fork that uses a slightly adapted version of
> dix/dixfonts.c:doListFontsAndAliases() for xorg-xserver.
>
> Generally both the original and my version are working like this:
> - init using FontFileStartListFonts(...,&private)
> - loop until FontFileListNextFontOrAlias(...,private) returns BadFontName
>
> The private data is malloc'ed by FontFileStartListFonts() and freed
> only by FontFileListNextFontOrAlias() at some point. Only in that case
> it will return BadFontName. In other words: If it returns BadFontName
> that also means the private data has been freed.
> FontFileListNextFontOrAlias() is the only place were the private data
> is freed within libXfont2.
>
> Now, the main difference in my case is that the loop is exited
> prematurely if some condition is true. In that case the private data
> will not have been freed. Valgrind find that and complains:
>
> ==15332== 2,500 (96 direct, 2,404 indirect) bytes in 6 blocks are
> definitely lost in loss record 324 of 342
> ==15332==    at 0x4C2DB8F: malloc (in
> /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
> ==15332==    by 0x5748B9E: FontFileStartListFonts (in
> /usr/lib/x86_64-linux-gnu/libXfont.so.1.4.1)
> ==15332==    by 0x5748C4A: FontFileStartListFontsAndAliases (in
> /usr/lib/x86_64-linux-gnu/libXfont.so.1.4.1)
> ==15332==    by 0x42859A: nxdoListFontsAndAliases (NXdixfonts.c:1163)
> ==15332==    by 0x42C0E0: nxOpenFont (NXdixfonts.c:1541)
> ==15332==    by 0x43392E: ProcOpenFont (NXdispatch.c:902)
> ==15332==    by 0x434585: Dispatch (NXdispatch.c:482)
> ==15332==    by 0x40EF77: main (main.c:355)
>
> The private data format contains some more allocated data so just
> calling free(private) is not freeing everything.
>
> AFAICS there's no function call in libXfont that can be used for
> freeing that private data. Internally libXfont uses a LFWIDataRec
> structure for the private data but that structure is not public.
>
> I can certainly clone the LFWIDataRec structure definition into my
> code and free the data myself this way but this could break anytime if
> libXfont changes the structure.
>
> So I am looking for a better way to free that data without having to
> run the loop to the end.
>
> Shouldn't libXfont offer some additional FontFileEndListFonts() function?
>
> Uli
>
> PS:
> - Source of FontFileListNextFontOrAlias
> https://gitlab.freedesktop.org/xorg/lib/libxfont/-/blob/master/src/fontfile/fontfile.c#L1079
> - Source of the code breaking the loop prematurely:
> https://github.com/ArcticaProject/nx-libs/blob/3.6.x/nx-X11/programs/Xserver/hw/nxagent/NXdixfonts.c#L1204