Date: Fri, 30 Jan 2015 14:10:24 -0800 From: Mark Johnston <markj@freebsd.org> To: Ed Maste <emaste@freebsd.org> Cc: freebsd-dtrace@freebsd.org Subject: Re: removing USDT's libelf dependency Message-ID: <20150130221024.GA2731@charmander.picturesperfect.net> In-Reply-To: <CAPyFy2CzUKNrzX=s6p6nBDVs=oYbztJya=bU54WhBi34k4XvgQ@mail.gmail.com> References: <20150130061355.GA14037@raichu> <CAPyFy2CzUKNrzX=s6p6nBDVs=oYbztJya=bU54WhBi34k4XvgQ@mail.gmail.com>
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On Fri, Jan 30, 2015 at 10:15:09AM -0500, Ed Maste wrote: > On 30 January 2015 at 01:13, Mark Johnston <markj@freebsd.org> wrote: > > > > It turns out that GNU ld implements a similar feature, albeit in a more > > restrictive manner: a symbol named __start_<id> will have its value > > filled in with the address of the section named <id>. The catch is that > > <id> has to be a valid C identifier, and the DOF section is > > conventionally named ".SUNW_dof", which of course doesn't quite work. I > > propose using "SUNW_dof" instead so that we can make use of this linker > > feature and eliminate the libelf dependency. > > > > DOF sections have their own ELF section type (SHT_SUNW_dof), so > > anything that might want to find a DOF section should already be > > searching for that rather than the conventional name. Depending on this > > linker feature is also arguably a portability concern given that lld is > > on the horizon, but we depend on it already anyway - this feature is > > used in FreeBSD's linker set implementation (which in turn is used by > > our SDT code :). So I don't think this would introduce a new problem. > > I'm not worried about issues with lld here - we can make it meet our > requirements. I am a little worried that there will be code floating > around that does strcmp(foo, ".SUNW_dof") though. A quick search > turned up some examples. Most are copies of drti.c but I did find one > case in some markjdb Github repo :-) Heh, oops. > Even if we don't end up changing > .SUNW_dof I think we should convert these cases to use the section > type instead of name. I agree. In the case of drti.c, we look for both the section type and name. There isn't much else out there that looks for a DOF section. > > We could give GNU ld 2.17.50 and lld knowledge of .SUNW_dof and > __SUNW_dof. This looks like it would be straightforward, but at the > cost of carrying around different FreeBSD-specific changes. > > I think our best approach will be to find an approach that upstream > linker projects (GNU ld, gold, lld) are willing to support. But, until > or unless that happens I'd much rather we switch to SUNW_dof than > require linking against libelf. Ok. It looks like gold also supports the __start_<secname> feature from GNU ld; there's a thread about handling the case where <secname> starts with a period here: http://comments.gmane.org/gmane.comp.gnu.binutils/61852, though it didn't really go anywhere. lld doesn't support the feature AFAICS. For now I'd rather just change the section name. -Mark
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