Date: Fri, 18 Jul 2014 16:36:25 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Ian Lepore <ian@FreeBSD.org> Cc: hackers@freebsd.org Subject: Re: [CFR] Adding a function to rtld-elf.so, how to handle Symbol.map? Message-ID: <20140718133625.GL93733@kib.kiev.ua> In-Reply-To: <1405689839.1312.148.camel@revolution.hippie.lan> References: <1405545833.1312.84.camel@revolution.hippie.lan> <20140717004537.GE93733@kib.kiev.ua> <1405616990.1312.111.camel@revolution.hippie.lan> <20140717172910.GH93733@kib.kiev.ua> <1405642661.1312.135.camel@revolution.hippie.lan> <20140718081455.GI93733@kib.kiev.ua> <1405689839.1312.148.camel@revolution.hippie.lan>
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--B+Izp8cWEYSQgag+ Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Fri, Jul 18, 2014 at 07:23:59AM -0600, Ian Lepore wrote: > On Fri, 2014-07-18 at 11:14 +0300, Konstantin Belousov wrote: > > On Thu, Jul 17, 2014 at 06:17:41PM -0600, Ian Lepore wrote: > > > On Thu, 2014-07-17 at 20:29 +0300, Konstantin Belousov wrote: > > > > On Thu, Jul 17, 2014 at 11:09:50AM -0600, Ian Lepore wrote: > > > > > On Thu, 2014-07-17 at 03:45 +0300, Konstantin Belousov wrote: > > > > > > On Wed, Jul 16, 2014 at 03:23:53PM -0600, Ian Lepore wrote: > > > > > [snip] > > >=20 > > > > > I did some looking around and Netbsd, Android, and Risc Os all > > > > > implemented this in their dynamic loaders, so that seemed like th= e way > > > > > to go. Android actually puts a function with this __gnu name in = its > > > > > libc, but all that function does is calls dl_unwind_find_exidx() = which > > > > > is implemented in their loader. > > > > >=20 > > > > > I've just discovered that the arm unwind support code that will a= rrive > > > > > as part of clang 3.5 appears to assume the Android way of things = unless > > > > > __LINUX__ is defined, so maybe it would be good to follow that mo= del > > > > > ourselves and add a dl_unwind_find_exidx() stub to libc/gen/dlfcn= =2Ec and > > > > > name the new implementation in ld-elf to match. > > > > I think that Android/__LINUX__ combination does the right thing, by > > > > providing the symbol in libc. A libc implementation does not need a= ny > > > > additional service from rtld, except already existing _rtld_addr_ph= dr(). > > > >=20 > > >=20 > > > Android provides a stub of dl_unwind_find_exidx() in libdl and the > > > shared-object implementation in the dynamic linker. What it puts in > > > libc is the __gnu_Unwind_Find_exidx() symbol, which just calls through > > > to the dl_unwind_find_exidx() implementation in the dynamic linker. > > >=20 > > > That aside, I've reworked my code so it all lives in libc instead of > > > rtld, as you suggested. It seems to work fine, and I guess I'm agnos= tic > > > about whether we're exporting a new function from libc versus rtld. = It > > > seems a bit strange to me to have just one dl_something() function wi= th > > > its shared/dynamic implementation in libc, while all the other functi= ons > > > with dl-prefix names are implemented in rtld. But not so weird that > > > it's a big deal. > > The new patch is fine with me. =20 > >=20 > > Could you, please, comment why did you decided to export the > > dl_unwind_find_exidx alias ? It was absent in the original patch, > > and from your description, it seems to be an implementation detail > > on Linux. >=20 > I think you might have misunderstood what I said earlier. According to > comments in some clang 3.5 sources I saw, the clang project considers > dl_unwind_find_exidx() to be "the BSD interface" for finding the exidx > data. They fall back to the gnu name only when clang is compiled with > __LINUX__ defined. By providing the functionality with this name, clang > 3.5 will just work right on freebsd without needing to be modified to > also use the gnu name when __FreeBSD__ is defined. >=20 > Android and Netbsd provide dl_unwind_find_exidx(); I haven't checked > other BSDs. It certainly is a better name for an interface shared by > different toolchains than __gnu_Unwind_Find_exidx(), although we do need > to also provide that symbol for anyone using gcc. Yes, I indeed misunderstood your description, thank you for the clarificati= on. I.e. clang on Linux and gcc use __gnu_Unwind_Find_exidx(), while (future) clang on non-Linuxes uses dl_unwind_find_exidx() ? And there is no ABI statement on the symbol, right ? --B+Izp8cWEYSQgag+ Content-Type: application/pgp-signature -----BEGIN PGP SIGNATURE----- Version: GnuPG v2 iQIcBAEBAgAGBQJTySLYAAoJEJDCuSvBvK1BpEkP/jcFWhlQVQuAiRAmPQrt/atB HXwkgOwyeNz+qPCfC6KylSVcYL7o5dtJBz6MeZjIBjLmzDX1K6IExC2wCWrcZ8mn Gsx7aN5j6dPEmSd7LMLOeqA3l/O9+oDNdH7GU0AewnMdPFxPbV4OMOyEAMxgQtwi c2sV/H5wKmbGM1yenAgVlkx5rrYOJ3aCg78fvTBqSCzBzbDjgDCJeoPuH0+Q0Yn/ XpWsHbxXKykrdmESoEE/WeWLSgKBObxgVCoelsPWjP/1wsdLjMwkPmsJACBrkc6P +dthlxW8xXTRptq2GRP6Fzw/wtiXY0HuKAZ1WtVX6BNq4DjbrsxYG7H0tg5qdqB3 WLk6IIUUrvH7kl5J8X0vz6k+7WTTLQNP0NJA3qsAS0WuhUf9tYvUKSFamPqOueyE WraA9/5xu2JwqM+wtMUCrh12mJNAPdyXgEgjpoR/0q2eCo4hZhKI5HKgEUSzwqvT RwSJ9zY9DFliL8cBUx29GZ31NmkDqYuk8qD12FAdFHpACn05iArpfCxMF2BejefU DJayiYLK/6khGSRe/tUTQwtmnE1HIpr2I8UkrP2hwhDnsqTJ5NLxZDbvwNzZJH/h w5ouUoxKAXZktCAkYdDw/gf6DwJkqXY7J8dyT0C8ooZtRemDcVsSaaMgnQu6u6Wb HEZLRiA6j/U2rxlmRHD0 =el5q -----END PGP SIGNATURE----- --B+Izp8cWEYSQgag+--
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