Date: Wed, 25 Jan 2017 14:59:01 -0500 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: Re: Need to zero pads in msghdr On Wed, Jan 25, 2017 at 11:46:44AM -0800, Andrei Vagin wrote: > On Wed, Jan 25, 2017 at 11:40 AM, Szabolcs Nagy <nsz@...t70.net> wrote: > > * Andrei Vagin <avagin@...il.com> [2017-01-25 10:56:22 -0800]: > >> On Wed, Jan 25, 2017 at 8:42 AM, Andrei Vagin <avagin@...il.com> wrote: > >> > In this patch > >> > http://git.musl-libc.org/cgit/musl/commit/arch/x86_64/bits/socket.h?id=7168790763cdeb794df52be6e3b39fbb021c5a64 > >> > you suppose that the kernel ignores the upper 32 bits of msg_iovlen, > >> > but it doesn't, so pads in msghdr structures have to be zeroed before > >> > calling sendmsg and recvmsg syscalls. > >> > >> Actually the problem is a bit different. In CRIU we use the msghdr > >> structure from musl-libc, but in some cases we have to call raw system > >> calls. We don't expect to have pads in structures and so we don't zero > >> them. > > > > why do you need a raw syscall? > > We inject our code into processes which are going to be dumped: > https://criu.org/Parasite_code > > And on restore we have to unmap old libc to restore process mappings. > > > > > (i think if you do raw syscalls you should use > > your own linux syscall wrappers including typedefs > > and macro defines, not libc ones, because the libc > > can and does do all sorts of remapping of things to > > workaround various mismatches between the posix > > library api it provides and the linux syscall abi) > > We know about this risk, but before this day we executed out test for > glibc and it worked for everyone. Now we need think how to resolve the > problem. While you may hit problems elsewhere, I think there's a clean solution here, just pre-zeroing the structure so that any possible padding is zero. It's unfortunate that this is needed but it comes from the kernel getting the structure definition wrong. Rich
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