On Sun, Sep 18, 2022 at 11:57 AM Ramon Rüttimann
<ramon.ruttim...@snyk.io> wrote:
>
> Mh, this is interesting. When I build a test binary with buildmode=pie, I
> even get an ELF section called `.data.rel.ro.gopclntab`.
>
> However, in the containerd binary, this section does not exist.There also is
> no `.gosymtab` section, as that is easily wiped by the `ldflags`.
>
> > It is getting folded into the
> > .data.rel.ro section. You can still find the information by looking
> > at the symbol table: the .gopclntab data is between runtime.pclntab
> > and runtime.epclntab.
>
>
> I'm not sure I follow.
> If I understand correctly, this means that what I see in my test binary as
> `.data.rel.ro.gopclntab` is a subsection "within" the `.data.rel.ro` section.
> Further, as the containerd binary has a `.data.rel.ro` section, at some
> offset within that data there should be a PCLN table?
>
> If so, two questions arise:
> - How do I find that offset? By looking for the magic string for the PCLN
> table?
Look in the symbol table for the runtime.pclntab and runtime.epclntab
symbols. The values of those symbols indicate the address of the
pclntab.
> - Why is that subsection not listed / present in the containerd binary?
Presumably because the system linker is merging all the .data.rel.ro.*
input sections into a single .data.rel.ro output section.
> PS: If you have some literature / documents on the Go specifics for this, I'd
> be curious to read them.
I don't know of any.
Ian
> On 18 Sep 2022, at 04:51, Ian Lance Taylor <i...@golang.org> wrote:
>
> On Sat, Sep 17, 2022 at 2:48 PM 'Ramon Rüttimann' via golang-nuts
> <golang-nuts@googlegroups.com> wrote:
>
>
> Sadly this does not seem to be the case, for example the nightly artifacts
> from containerd, which can be found here.
> A `go version -m <binary>` lists all deps & shows the Go version 1.19.1, but
> there is now PCLN table to be found...
> The binary seems to be created through the following go build command:
>
> go build -gcflags=-trimpath=/home/runner/work/containerd/containerd/src
> -buildmode=pie -o bin/containerd -ldflags '-X
> github.com/containerd/containerd/version.Version=290ef2b -X
> github.com/containerd/containerd/version.Revision=290ef2b43ff1b824e56d0adaebf621e053de6e86
> -X
> github.com/containerd/containerd/version.Package=github.com/containerd/containerd
> -s -w ' -tags "urfave_cli_no_docs" ./cmd/containerd
>
> Local testing confirms that with `-buildmode=pie` ("Position Independent
> Executable") the PCLN table is not being added to the binary. Why?
> I'm not familiar with these executables, not sure how this would affect the
> PCLN.
>
>
> Interesting. I agree: with -buildmode=pie the .gopclntab section does
> not appear in the final executable. It is getting folded into the
> .data.rel.ro section. You can still find the information by looking
> at the symbol table: the .gopclntab data is between runtime.pclntab
> and runtime.epclntab.
>
> Ian
>
>
> On Friday, September 16, 2022 at 9:36:53 PM UTC+2 Ian Lance Taylor wrote:
>
>
> On Fri, Sep 16, 2022 at 12:13 PM Ramon Rüttimann
> <ramon.ru...@gmail.com> wrote:
>
>
> I am currently looking into what a built Go binary may contain and have found
> some confusing things around the `.gopclntab` ELF section, also known as the
> PCLN table.
>
> At first, I assumed that the PCLN table is present in all binaries, no matter
> how they are built. I've done some tests with stripped, trimmed and CGo
> binaries and have found it to be present.
> However, this article contains an interesting statement:
>
> For ordinary unstripped Go binaries, this debugging information is in the
> .gopclntab and .gosymtab ELF sections of the binary, and can be read out with
> debug/elf/File.Section() and Section.Data(). Unfortunately, Go binaries that
> use cgo do not have these Go ELF sections. As mentioned in Building a better
> Go linker:
>
> For “cgo” binaries, which may make arbitrary use of C libraries, the Go
> linker links all of the Go code into a single native object file and then
> invokes the system linker to produce the final binary.
>
> This linkage obliterates .gopclntab and .gosymtab as separate ELF sections. I
> believe that their data is still there in the final binary, but I don't know
> how to extract them.
>
> I have also found such binaries in the wild, like the `containerd` binary or
> some tools from Docker as well. I've tried to decipher how the containerd
> binary is being built (based on this Makefile), but I am not really familiar
> enough with Make to decipher this.
>
> This S/O post has asked the same question, but the answer doesn't match what
> I've seen and also what that article claims.
>
> Can someone shed some lights on how one can build CGo binaries without the
> `.pclntab` section? Also, what happens to stacktraces in such a case?
>
>
> I believe that at least as of Go 1.19 the .gopclntab section exists
> even if the binary is built with cgo, and the .gosymtab section is
> empty in any case.
>
> Ian
>
>
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