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From: Josef W. <Jos...@gm...> - 2003-03-10 13:17:03
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On Sunday 09 March 2003 17:37, Nicholas Nethercote wrote:
> On Fri, 7 Mar 2003, Josef Weidendorfer wrote:
> > > Thinking about it some more, the only reason I can think of why
> > > valgrind even needs its own malloc() is this:
>
> Thinking more, another important reason is this: for memcheck/addrcheck,
> having the redzones around malloc'd blocks allows them to catch
> over/under-runs.
Yes, but that's not needed for a pure tracing/profiling skin like cachegrind,
too.
> > As I see, a skin is able to implement it's own GLIBC wrapper functions,
> > no matter if they are called malloc() or perhaps open(), simply by
> > defining the function in the skin.
> > This is simply another (but static) way of defining the behaviour of a
> > skin. E.g. in vg_skin.h, do a
> >
> > #define REDIRECT1(fn,rType,pType1) \
> > rType fn(pType1 p1) { return VG_(fn)(p1); }
> >
> > and in the skin, put
> >
> > REDIRECT1(malloc,void*,size_t)
> >
> > Of course it would be good to be able to check at runtime if the skin
> > defined its own wrapper for a function: Use dlsym(RTLD_DEFAULT,"malloc")
> > and check if the address is defined inside of the skin. You can issue a
> > warning if no wrapper was defined when e.g. "new_mem_heap" is requested.
>
> Well, dlsym() is not available for use within Valgrind, unless someone
> implements VG_(dlsym)(). I think.
Ok. Next try:
#define REDIRECT1(fn,rType,pType1) \
rType fn(pType1 p1) { return VG_(fn)(p1); } \
Boolean VG_(fn##_wrapper_defined) = True;
For all symbols where checking for a wrapper is needed, put e.g.
Boolean VG_(malloc_wrapper_defined) = False;
into valgrind core. As the skin lib is ahead of the valgrind lib in
LD_PRELOAD, you will get the "True" value if the wrapper is defined
(Perhaps the boolean in valgrind core must be a weak symbol).
> It's an interesting idea, but feels a bit messy to me... there's one more
> thing for a skin to get wrong (ask for "new_mem_heap" but forget the
> REDIRECT). Doing things statically with macros is a bit hacky too, but
> maybe unavoidable.
Best is to make the "ask for new_mem_heap but forget the REDIRECT" a fatal
error, checked at startup.
> But I do like the idea of the default behavour being "glibc malloc(), no
> wrapper" and that a skin should have to request different behaviour...
> kind of a "principle of least interference" with the original program.
Especially as KDE comes with a malloc implementation of its own. With current
valgrind, cachegrind can't check if the "KDE fast malloc" is really faster
than the glibc provided one.
>
> Here's my ideal scenario:
>
> i. default is no wrapper + glibc malloc()
>
> ii. skin can specify with a need if it wants to use Valgrind's malloc()
> (eg. needs_redzones, or needs_special_malloc, or something)
>
> iii. core decides whether a malloc() wrapper is needed (eg. because
> new_mem_heap is being tracked, or --trace-malloc=yes is set, or
> core errors or shadow chunks are needed
>
> iv. if wrapper is needed, core calls glibc malloc() or Valgrind
> malloc() depending on the needs_redzones/whatever need
>
>
> I don't think all this is attainable. Esp (iii), since the conditions
> there are all dynamic, but the overwriting by Valgrind of glibc's malloc()
> symbol must be done statically.
Valgrind core must be able to check whether the skin provides a wrapper.
This is best done with a second symbol definition, and relying on the runtime
linker for this (see above), as this is the same mechanism.
> I think wrapper + glibc() malloc might be attainable with the --wrap
> linker option, but I'm not certain.
I'm not sure about this. A program to be run under supervison of valgrind is
already linked; you can't change symbol names in it.
But GLIBC is prepared for wrapping its symbols: To get the real malloc, use
"__libc_malloc". As said above, KDE does the same wrapping with its "fast
malloc implementation" as we want to do here. You can have a look at the
code: Use the KDE CVS web frontend and look into "kdelibs/kdecore/malloc".
Josef
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