Date: Wed, 9 Sep 2015 14:48:22 -0400 From: Zack Weinberg <zackw@...ix.com> To: Rich Felker <dalias@...c.org> Cc: gcc@....gnu.org, GNU C Library <libc-alpha@...rceware.org>, musl@...ts.openwall.com Subject: Re: Compiler support for erasure of sensitive data On 09/09/2015 01:13 PM, Rich Felker wrote: > On Wed, Sep 09, 2015 at 12:47:10PM -0400, Zack Weinberg wrote: >> On Wed, Sep 9, 2015 at 12:42 PM, Rich Felker <dalias@...c.org> wrote: >>> You're making this harder than it needs to be. The "m" constraint is >>> the wrong thing to use here. Simply use: >>> >>> __asm__(""::"r"(ptr):"memory"); >> >> Please review my earlier conversation with Adhemerval on exactly this point. > > My understanding is that you consider this a "big hammer". Does that > really matter if the intent is that it only be used in isolated, > sensitive contexts? Are you just unhappy with the performance cost, or > concerned that the clobber will cause more spilling of sensitive data? Please review *all* of my earlier conversation with Adhemerval, in particular the bit where I compiled libressl three different ways and analyzed the assembly dumps. I'm sure there's more to be said on the topic, but *starting* from there. > the hack with the "m" constraint is wrong and easily fixed It's not wrong; it is in fact the documented way to express a fixed-size read access to one block of memory. Look for "ten bytes of a string" within https://gcc.gnu.org/onlinedocs/gcc-4.8.1/gcc/Extended-Asm.html (sorry, there don't appear to be anchors). It merely doesn't work in C++, with Clang, or (maybe) with a block of memory whose size cannot be determined at compile time. zw
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