linux-kernel - Re: vdso feature requests from the Go people

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Message-ID: <CALCETrX9Og2LR3d8HZrcn4uGAgLfxbv51LK4Tsq-gDZKGpsjYw@mail.gmail.com>
Date:	Thu, 12 Jun 2014 22:23:44 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, Russ Cox <rsc@...ang.org>,
	Ian Taylor <iant@...ang.org>
Subject: Re: vdso feature requests from the Go people

On Thu, Jun 12, 2014 at 10:15 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 06/12/2014 09:36 PM, Andy Lutomirski wrote:
>>
>> If we were to implement both, maybe we'd actually want to provide
>> something like:
>>
>> struct vdso_entry {
>>   unsigned long vdso_entry_struct_size;  /* so we can add fields later on */
>>   void *func;
>>   unsigned int max_stack;  /* zero if not known */
>> };
>>
>
> How different is this really from the .dynsym table -- other than that
> you have to find the latter via the dynamic segment?
>
> The hash part is just to speed up lookup...

As far as I know, there's no reliable way to just read the dynsym
table -- the thing doesn't have a specified length, which is what
broke Go in the first place.

Parsing the ELF dynamic tables is kind of annoyingly complicated, and
understanding the format is a real PITA -- the documentation I've been
able to find is outright terrible.

--Andy
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