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Date: Sun, 17 Oct 2021 09:50:10 +0200 From: Christophe Leroy <christophe.leroy@...roup.eu> To: Kees Cook <keescook@...omium.org> Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>, Paul Mackerras <paulus@...ba.org>, Michael Ellerman <mpe@...erman.id.au>, Andrew Morton <akpm@...ux-foundation.org>, "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>, Helge Deller <deller@....de>, Arnd Bergmann <arnd@...db.de>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org, linux-ia64@...r.kernel.org, linux-parisc@...r.kernel.org, linux-arch@...r.kernel.org, linux-mm@...ck.org Subject: Re: [PATCH v2 11/13] lkdtm: Fix lkdtm_EXEC_RODATA() Le 16/10/2021 à 08:41, Christophe Leroy a écrit : > > > Le 15/10/2021 à 23:32, Kees Cook a écrit : >> On Thu, Oct 14, 2021 at 07:50:00AM +0200, Christophe Leroy wrote: >>> Behind its location, lkdtm_EXEC_RODATA() executes >>> lkdtm_rodata_do_nothing() which is a real function, >>> not a copy of do_nothing(). >>> >>> So executes it directly instead of using execute_location(). >>> >>> This is necessary because following patch will fix execute_location() >>> to use a copy of the function descriptor of do_nothing() and >>> function descriptor of lkdtm_rodata_do_nothing() might be different. >>> >>> And fix displayed addresses by dereferencing the function descriptors. >>> >>> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu> >> >> I still don't understand this -- it doesn't look needed at all given the >> changes in patch 12. (i.e. everything is using >> dereference_function_descriptor() now) > > dereference_function_descriptor() only deals with the function address, > not the function TOC. > > do_nothing() is a function. It has a function descriptor with a given > address (address of .do_nothing) and a given TOC, say TOC1. > > lkdtm_rodata_do_nothing() is another function. It has its own function > descriptor with a given address (address of .lkdtm_rodata_do_nothing) > and a given TOC, say TOC2. > > If we use execute_location(), it will copy do_nothing() function > descriptor and change the function address to the address of > lkdtm_rodata_do_nothing(). So it will call lkdtm_rodata_do_nothing() > with TOC1 instead of calling it with TOC2. > >> >> Can't this patch be dropped? > > It is likely that the TOC will be the same for both functions, and > anyway those functions are so simple that they don't use the TOC at all, > so yes it would likely work without this patch but from my point of view > it is incorrect to call one function with the TOC from the descriptor of > another function. > > If you thing we can take the risk, then I'm happy to drop the patch and > replace it by > > execute_location(dereference_function_descriptor(lkdtm_rodata_do_nothing), CODE_AS_IS) > Once we have patch 12 EXEC_RODATA works well on powerpc without this patch so I will drop this patch for now and will propose something else as a follow-up to my series. Christophe
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