linux-kernel - Re: [RFC] New codectl(2) system call for sframe registration

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Message-ID: <afdce562-d434-4137-8682-bf92e9e373b3@oracle.com>
Date: Tue, 22 Jul 2025 14:57:22 -0700
From: Indu Bhagat <indu.bhagat@...cle.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: "Jose E. Marchesi" <jemarch@....org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
        bpf@...r.kernel.org, x86@...nel.org,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrii Nakryiko <andrii@...nel.org>,
        Beau Belgrave <beaub@...ux.microsoft.com>,
        Jens Remus
 <jremus@...ux.ibm.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>, Florian Weimer <fweimer@...hat.com>,
        Sam James <sam@...too.org>, Brian Robbins <brianrob@...rosoft.com>,
        Elena Zannoni <elena.zannoni@...cle.com>
Subject: Re: [RFC] New codectl(2) system call for sframe registration

On 7/22/25 2:13 PM, Steven Rostedt wrote:
> On Tue, 22 Jul 2025 14:04:37 -0700
> Indu Bhagat <indu.bhagat@...cle.com> wrote:
> 
>> Yes and No.  The offset at which the text is loaded is _one_ part of the
>> information to "fill in the blanks".  The other part is what to do with
>> that information (text_vma) or how to relocate the SFrame section itself
>> a.k.a. the relocation entries.  To know the relocations, one will need
>> to get access to the respective relocation section, and hence access to
>> the ELF section headers.
> 
> You mean to find where in the sframe section itself that needs to be update?
> 

Correct.  Each relocation entry carries pieces of information like :what 
is the location to update, how many bytes to update and what is the 
calculation to use i.e., the relocation type.

> OK, that makes sense. So sframes does need to still be in an ELF file for
> its own relocations and such.
> 
> It will be interesting on how to do compression and on-demand page loading.
> 

Right, its an open item.

Compression (SHF_COMPRESSED) for non SHF_ALLOC sections  is doable.  In 
fact, debug sections use it already.

The tricky part is SHF_ALLOC and SHF_COMPRESSED, which is what SFrame 
may need.  This is currently not allowed in ELF.  Some previous 
discussion here https://groups.google.com/g/generic-abi/c/HUVhliUrTG0. 
Not sure if things have evolved since.

> There would need to be a table as well that will denote where in the
> decompressed pages that relocations need to be performed.
> 
> -- Steve