Date: Fri, 18 Jul 2014 07:49:14 -0600 From: Ian Lepore <ian@FreeBSD.org> To: Konstantin Belousov <kostikbel@gmail.com> Cc: hackers@freebsd.org Subject: Re: [CFR] Adding a function to rtld-elf.so, how to handle Symbol.map? Message-ID: <1405691354.1312.152.camel@revolution.hippie.lan> In-Reply-To: <20140718133625.GL93733@kib.kiev.ua> 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> <20140718133625.GL93733@kib.kiev.ua>
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On Fri, 2014-07-18 at 16:36 +0300, Konstantin Belousov wrote: > 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] > > > > > > > > > > I did some looking around and Netbsd, Android, and Risc Os all > > > > > > implemented this in their dynamic loaders, so that seemed like the 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. > > > > > > > > > > > > I've just discovered that the arm unwind support code that will arrive > > > > > > 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 model > > > > > > ourselves and add a dl_unwind_find_exidx() stub to libc/gen/dlfcn.c 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 any > > > > > additional service from rtld, except already existing _rtld_addr_phdr(). > > > > > > > > > > > > > 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. > > > > > > > > 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 agnostic > > > > 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 with > > > > its shared/dynamic implementation in libc, while all the other functions > > > > 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. > > > > > > 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. > > > > 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. > > > > 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 clarification. > 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 ? Right, except I don't understand what you are asking in your last sentence. -- Ian
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