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Re: Problem report #16

From: Kenichi Handa
Subject: Re: Problem report #16
Date: Thu, 13 Apr 2006 20:21:33 +0900

In article <address@hidden>, "Stuart D. Herring" <address@hidden> writes:

> There's actually several possible problems here.
>> > At conditional (1): "face != 0" taking false path
>> >
>> > 667          if (face)
>> > 668            id = face->fontset;

> This is a red herring -- face is reassigned later.

Why? face is reassigned as a temporary value folder just
before return (L686).

>> > At conditional (2): "id < 0" taking true path
>> >
>> > 669          if (id < 0)
>> > 670            fontset = Qnil;
>> > 671          else
>> > 672            fontset = FONTSET_FROM_ID (id);

> Here's one thing I don't like: FONTSET_FROM_ID doesn't do any safety
> checks on id.  Can we trust this function to always get reasonable id
> values?

That's the other way round.  FONTSET_FROM_ID should be
called only when ID is valid.  And, here, it is assured;
that is to say, if face != NULL, face->fontset must be -1 or
valid, and if face == NULL, the given ID must be -1 or

>> > 682                  int face_id = XINT (elt);
>> > 684                  xassert (face_id == face->id);

> Dan wrote (although not here in his message):
>> This problem could happen if fs_load_font was called with face=NULL
>> and id>0. Can that happen?

> Here's where Dan's point is relevant: if face==NULL and id>0, then it
> seems quite possible for this line to be reached...

>> > 685                  face = FACE_FROM_ID (f, face_id);

There is just one place calling fs_load_font() with
face==NULL and id>=0; list_fontsets() in fontset.c.  But
that function calls fs_load_font() (via FS_LOAD_FONT macro)
only if BASE_FONTSET_P (fontset) is true, in which
case, this line is never reached.

> ...just before assigning face, so it'd still be NULL.  But there's more:
> FACE_FROM_ID can fail and return NULL:

>> > Event var_deref_op: Variable "face" tracked as NULL was dereferenced.
>> > Also see events: [var_compare_op]
>> >
>> > 686                  return (*get_font_info_func) (f, face->font_info_id);

> So this part is dangerous if and only if face_id, derived from the
> fontset, can be messed up.

face_id derived from the fontset should be always valid.  If
not, that means there's a bug somewhere else.

Anyway, fs_load_font() is now very complicated because of
repeated changes in the long history.  So, I rewrote
fontset.c in emacs-unicode-2 branch.  It will be helpful if
the similar report is issued on that branch because it
contains many new codes.

Kenichi Handa

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