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Re: [Qemu-devel] [PATCH v2 01/47] acpi: introduce AML composer aml_appen


From: Michael S. Tsirkin
Subject: Re: [Qemu-devel] [PATCH v2 01/47] acpi: introduce AML composer aml_append()
Date: Mon, 26 Jan 2015 18:17:55 +0200

On Mon, Jan 26, 2015 at 04:34:01PM +0100, Andrew Jones wrote:
> On Mon, Jan 26, 2015 at 04:09:20PM +0100, Igor Mammedov wrote:
> > On Mon, 26 Jan 2015 10:57:21 +0100
> > Igor Mammedov <address@hidden> wrote:
> > 
> > > On Sat, 24 Jan 2015 18:33:50 +0200
> > > "Michael S. Tsirkin" <address@hidden> wrote:
> > > 
> > > > On Fri, Jan 23, 2015 at 06:56:20PM +0100, Igor Mammedov wrote:
> > > > > On Fri, 23 Jan 2015 15:55:11 +0200
> > > > > "Michael S. Tsirkin" <address@hidden> wrote:
> > > > > 
> > [...]
> > > > > I refuse to give up on cleaner and simpler API yet :)
> > > > > 
> > > > > > 
> > > > > > 
> > > > > > Your patches are almost there, they are pretty clean, the only 
> > > > > > issue I
> > > > > > think is this passing of AcpiAml by value, sometimes freeing buffer 
> > > > > > in
> > > > > > the process, sometimes not.
> > > > > Currently buffer is allocated by API and is always freed whenever
> > > > > it's passed to another API function.
> > > > > That's why it makes user not to care about memory mgmt.
> > > > > 
> > > > > The only limitation of it is if you store AcpiAml return value into 
> > > > > some
> > > > > variable you are responsible to use it only once for passing to 
> > > > > another API
> > > > > function. Reusing this variable's value (pass it to API function 
> > > > > second time)
> > > > > would cause cause use-after-free and freeing-freed bugs.
> > > > > Like this:
> > > > > AcpiAml table = acpi_definition_block("SSDT",...);
> > > > > AcpiAml scope = acpi_scope("PCI0");
> > > > > aml_append(&table, scope); // <- here scope becomes invalid
> > > > > // a bug
> > > > > aml_append(&table, scope); // use-after-free + freeing-freed bugs
> > > > > 
> > > > > There are several approaches to look for resolving above issues:
> > > > > 1. Adopt and use memory mgmt model used by GTK+
> > > > >    in nutshell: 
> > > > > http://www.cs.hunter.cuny.edu/~sweiss/course_materials/csci493.70/lecture_notes/GTK_memory_mngmt.pdf
> > > > >    In particular adopt behavior of GInitiallyUnowned usage model
> > > > > 
> > > > >    that will allow to keep convenient chained call style and if 
> > > > > necessary
> > > > >    reuse objects returned by API by explicitly 
> > > > > referencing/dereferencing
> > > > >    them if needed.
> > > > 
> > > > Hmm, it's still easy to misuse. I think I prefer option 2 below.
> > > That's basically what we have/use in QOM with object_new(FOO) + 
> > > object_unref()
> > > I have no idea why we invented our own Object infrastructure
> > > when we could just use GObject one from already used glib.
> > > 
> > > > 
> > > > > 2. It's possible to drop freeing inside API completely and
> > > > >    record(store in list) every new object inside a table context.
> > > > >    When table is constructed, list of created objects could be
> > > > >    safely freed.
> > > > >    With that it would be safe to reuse every AcpiAml object
> > > > >    and avoid free-after-use issues with limitation that created
> > > > >    AcpiAml objects shouldn't be used after table was closed.
> > > > >    It should cover all practical use of API, i.e. no cross
> > > > >    table AcpiAml objects.
> > > > 
> > > > So each aml_alloc function gets pointer to this list,
> > > > and adds the new element there.
> > > > Eventually we do free_all to free all elements,
> > > > so there isn't even an aml_free to mis-use.
> > > I'm thinking a little bit different about implementation though.
> > > I still don't like the use of explicit alloc/free being called
> > > by API user since it doesn't allow chained API calls and
> > > I think it's unnecessary complication see below why.
> > > 
> > > Here is what's true about current API and a I'd like to with it:
> > > 
> > >   1. Every API call (except aml_append) makes aml_alloc(), it's just
> > >      like a wrapper about object_new(FOO). (current + new impl.)
> > > 
> > >   2 Every API call that takes AML type as input argument
> > >   2.1 consumes (frees) it (current impl.)
> > >       (it's easy to fix use after free concern too,
> > >        just pass AML by pointer and zero-out memory before it's freed
> > >        and assert whenever one of input arguments is not correct,
> > >        i.e. it was reused second time)
> > >       There is no need for following steps after this one.
> > >   2.2 takes ownership of GInitiallyUnowned and adds it to its list
> > >       of its children.
> > >   3. Free children when AML object is destroyed (i.e. ref count zero)
> > >      That way when toplevel table object (definition block in 42/47)
> > >      is added to ACPI blob we can unref it, which will cause
> > >      its whole children tree freed, except for AML objects where
> > >      API user explicitly took extra reference (i.e. wanted them
> > >      to reuse in another table)
> > > 
> > > I'd prefer:
> > >  *  2.1 way to address your current concern of use-after-free
> > >     as the most simplest one (no reuse is possible however)
> > > or
> > >  * follow already used by QEMU QOM/GObject pattern of
> > >    implicit alloc/free
> > > 
> > > since they allow to construct AML in a more simple/manageable way i.e.
> > >  
> > >   aml_append(method,
> > >       aml_store(aml_string("foo"), aml_local(0)))
> > >   );
> > > 
> > > v.s. explicit headache of alloc/free, which doesn't fix
> > >      use-after-free anyway and just adds more boiler plate
> > >      plus makes code har to read read
> > > 
> > >   str = aml_alloc();
> > >   aml_string(str, "foo");
> > >   loc0 = aml_alloc();
> > >   aml_local(loc0, 0);
> > >   store = aml_alloc();
> > >   aml_store(store, str, loc0);
> > >   aml_append(method, store);
> > >   aml_free(store);
> > >   aml_free(loc0);
> > >   aml_free(str);
> > 
> > Here is a compromise what I and Michael came to on a phone call:
> > 
> > Externally API usage would look like:
> > 
> > AmlAllocList *p = some_list_alloc();
> > 
> > Aml *ssdt = aml_def_block(p, "SSDT", ...);
> > Aml *dev = aml_device(p, "PCI0");
> > aml_append(dev,
> >     aml_name_def(p, "_STA", aml_int(p, 0xF /* present */))
> > );
> > aml_append(ssdt, dev);
> > 
> > aml_append(acpi_tables_blob, ssdt);
> > 
> > free_aml_alloc_list(p);
> > 
> > 
> > Each of aml_foo() will take other Aml arguments by pointer.
> > Also every aml_foo(), except of aml_append() will allocate
> > Aml struct and return pointer to it and also add this pointer
> > into AmlAllocList which is passed as first argument to each
> > aml_foo() call.
> > aml_append() becomes nondestructive function and just adds
> > child(2nd arg) to the parent context (1st arg).
> > 
> > After API user is done with building table and pushed it
> > into tables blob, he/she calls free_aml_alloc_list() to free
> > all Aml objects created during process of building the table
> > content.
> 
> Hmm, passing 'p' around somewhat muddies an otherwise clean
> interface, but the concern with aml_append silently freeing
> memory still accessible by the caller is definitely valid. I
> only wonder how things would look with Igor's option 2.2 above.
> The caller still only needs to free the final table, but it
> also becomes safe to use the same object references multiple
> times before freeing the table. Using QOM also seems reasonable
> to me, as it appears it's the accepted way to do garbage
> collection in QEMU. Is it possible to do 2.2 with QOM?

I'd rather not go there: QOM was really invented for introspection, and
for long-lived heavy-weight objects.  And to me, code using QOM is
harder to understand than simple alloc/free. It's worth it where we need
the features it offers, e.g. it has run-time checks where we previously
just did a cast. But in this case I'd rather use something simpler and
with compile-time checks.



> > 
> > > 
> > > > 
> > > > Good idea! I think this will address the issue.
> > > > 
> > > > 
> > > > > 3. talloc implementation Amit've mentioned,
> > > > >    perhaps it might work since it allows to set destructors for
> > > > >    managed pointers. With this we might get clear abort when
> > > > >    dereferencing freed pointer see talloc_set()
> > > > 
> > > > 
> > > > I think it's a separate discussion. Maybe talloc is a good
> > > > allocator to use in qemu, but using a separate allocator
> > > > just for acpi generation would be an overkill.
> > > > 
> > > > > 
> > > > > > 
> > > > > > Just pass AcpiAml* everywhere, add APIs to allocate and free it
> > > > > > together with the internal buffer.
> > > > > > This makes it trivial to see that value is not misused:
> > > > > > just check it's between alloc and free - and that there are
> > > > > > no leaks - just check we call free on each value.
> > > > > > We can write a semantic patch to catch missing free calls,
> > > > > > it's easy.
> > > > > > 
> > > > > > 
> > > > > > > > 
> > > > > > > > > As for moving to to another file, during all this series 
> > > > > > > > > lowlevel
> > > > > > > > > build_(some_aml_related_costruct_helper)s are moved into this 
> > > > > > > > > file
> > > > > > > > > and should be make static to hide from user lowlevel helpers
> > > > > > > > > (including build_package).
> > > > > > > > > That will leave only high level API available.
> > > > > > > > > 
> > > > > > > > > TODO for me: make sure that moved lowlevel helpers are static
> > > > > > > > > 
> > > > > > > > > 
> > 
> > 



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