| Summary: | Extraneous FcPatternDestroy in XftFontOpenInfo leading to segfault | ||||||
|---|---|---|---|---|---|---|---|
| Product: | xorg | Reporter: | Gautam Iyer <gautam> | ||||
| Component: | Lib/Xft | Assignee: | Jeremy Huddleston Sequoia <jeremyhu> | ||||
| Status: | RESOLVED INVALID | QA Contact: | |||||
| Severity: | normal | ||||||
| Priority: | high | CC: | jeremyhu | ||||
| Version: | unspecified | ||||||
| Hardware: | All | ||||||
| OS: | Linux (All) | ||||||
| Whiteboard: | 2011BRB_Reviewed | ||||||
| i915 platform: | i915 features: | ||||||
| Attachments: |
|
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Created attachment 4493 [details]
C source that segfaults because of this bug.
Using guard malloc, I caught this:
Program received signal EXC_BAD_ACCESS, Could not access memory.
Reason: KERN_INVALID_ADDRESS at address: 0x00000001019f6ff0
0x000000010018fae0 in FcPatternDestroy (p=0x1019f6fe0) at fcpat.c:286
286 if (p->ref == FC_REF_CONSTANT)
(gdb) bt
#0 0x000000010018fae0 in FcPatternDestroy (p=0x1019f6fe0) at fcpat.c:286
#1 0x0000000100001a45 in main (argc=1, argv=0x7fff5fbff5c0) at test.c:82
Current language: auto; currently minimal
(gdb) list
281 FcPatternDestroy (FcPattern *p)
282 {
283 int i;
284 FcPatternElt *elts;
285
286 if (p->ref == FC_REF_CONSTANT)
287 {
288 FcCacheObjectDereference (p);
289 return;
290 }
malloc_history shows:
ALLOC 0x1019f6fe0-0x1019f6fff [size=32]: thread_7fff7cf91960 |start | main |
XftFontMatch | FcFontMatch | FcFontRenderPrepare | FcPatternCreate |
GMmalloc_zone_malloc_internal
----
FREE 0x1019f6fe0-0x1019f6fff [size=32]: thread_7fff7cf91960 |start | main |
XftFontOpenPattern | XftFontOpenInfo | FcPatternDestroy | GMfree |
GMmalloc_zone_free
So the problem is that it's being destroyed inside XftFontOpenPattern
Why are we destroying the pattern here?
for (font = (XftFontInt *) *bucket; font; font = (XftFontInt *)
font->hash_next)
if (XftFontInfoEqual (&font->info, fi))
{
if (!font->ref++)
--info->num_unref_fonts;
FcPatternDestroy (pattern);
return &font->public;
}
It dates back to the initial import.
Removing that FcPatternDestroy seems to fix the issue, but I'm surprised this
has lasted so long... I'll send a patch to xorg-devel.
From the mailing list: <alanc> I'm not sure, since if it passes that point, it saves the pattern in the newly created font's font->public.pattern and then does the FcPatternDestroy of it in XftFontDestroy. Perhaps we just need to document that the pattern passed to XftFontOpenInfo becomes the exclusive property of Xft and the caller is not allowed to use it any more? (Though it seems XftFontOpenInfo is missing from the function list in the Xft man page, so needs more love than that.) </alanc> <me> If that's the case, then this is a terrible API. If the function returns NULL, the caller has no information about the state of its passed pattern. It may have been deallocated and NULL returned during 'goto bail' or it may not have been deallocated and NULL returned because of invalid paramaters (currently just info == NULL). Since the function isn't documented, I wonder if the "fix" would be to copy pattern and write documentation to this effect. The problem would then be a leak for those expecting it to be freed, but that is certainly preferred, IMO. Also, the leak is easily fixed by requiring the new libXft and doing their own FcPatternDestroy. This confusion has caused problems in external projects who expect it to not be freed. The only non-xorg hit for XftFontOpenInfo in codesearch is actually a long comment debugging this issue and removing their FcPatternDestroy: http://google.com/codesearch#Lbvu5g5_Qs8/mrxvt-0.5.2/src/main.c&q=XftFontOpenInfo%20lang:%5Ec$&type=cs </me> Mass closure: This bug has been untouched for more than six years, and is not obviously still valid. Please reopen this bug or file a new report if you continue to experience issues with current releases. |
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Hi All, The following code snipped results in a segfault: xftfont = XftFontOpenPattern( dpy, match); XftFontClose( dpy, xftfont); xftfont = XftFontOpenPattern( dpy, match); XftPatternDestroy( dpy, match); I think the reason is as follows: In line 783 of xfreetype.c (Xft-2.1.7), the function XftFontOpenInfo frees the font pattern. This is done only if the pattern corresponds to a font that's already opened (cached). The user is (and should be) responsible for freeing the pattern passed to XftFontOpenPattern (which in turn calls XftFontOpenInfo). If Xft frees this pattern for the user, then all hell breaks lose (segfault). Also note, that in the above code snippet, commenting out the lines that close and reopen the font results in perfectly working code. Thanks, Gautam