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

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

Hi Mathieu,

On Mon, 21 Jul 2025 11:20:34 -0400
Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:

> Hi!
> 
> I've written up an RFC for a new system call to handle sframe registration
> for shared libraries. There has been interest to cover both sframe in
> the short term, but also JIT use-cases in the long term, so I'm
> covering both here in this RFC to provide the full context. Implementation
> wise we could start by only covering the sframe use-case.
> 
> I've called it "codectl(2)" for now, but I'm of course open to feedback.

Nice idea for JIT, but I doubt we need this for ELF.

> 
> For ELF, I'm including the optional pathname, build id, and debug link
> information which are really useful to translate from instruction pointers
> to executable/library name, symbol, offset, source file, line number.

For ELF file, does the kernel already know how to parse the elf header?
I just wonder what happen if user sends different information to the
kernel.

> This is what we are using in LTTng-UST and Babeltrace debug-info filter
> plugin [1], and I think this would be relevant for kernel tracers as well
> so they can make the resulting stack traces meaningful to users.
> 
> sys_codectl(2)
> =================
> 
> * arg0: unsigned int @option:
> 
> /* Additional labels can be added to enum code_opt, for extensibility. */
> 
> enum code_opt {
>      CODE_REGISTER_ELF,
>      CODE_REGISTER_JIT,
>      CODE_UNREGISTER,
> };
> 
> * arg1: void * @info
> 
> /* if (@option == CODE_REGISTER_ELF) */
> 
> /*
>   * text_start, text_end, sframe_start, sframe_end allow unwinding of the
>   * call stack.
>   *
>   * elf_start, elf_end, pathname, and either build_id or debug_link allows
>   * mapping instruction pointers to file, symbol, offset, and source file
>   * location.
>   */
> struct code_elf_info {
> :   __u64 elf_start;
>      __u64 elf_end;
>      __u64 text_start;
>      __u64 text_end;

What happen if there are multiple .text.* sections?
Or, does it used for each text section?

>      __u64 sframe_start;
>      __u64 sframe_end;
>      __u64 pathname;              /* char *, NULL if unavailable. */
> 
>      __u64 build_id;              /* char *, NULL if unavailable. */
>      __u64 debug_link_pathname;   /* char *, NULL if unavailable. */
>      __u32 build_id_len;
>      __u32 debug_link_crc;
> };
> 
> 
> /* if (@option == CODE_REGISTER_JIT) */
> 
> /*
>   * Registration of sorted JIT unwind table: The reserved memory area is
>   * of size reserved_len. Userspace increases used_len as new code is
>   * populated between text_start and text_end. This area is populated in
>   * increasing address order, and its ABI requires to have no overlapping
>   * fre. This fits the common use-case where JITs populate code into
>   * a given memory area by increasing address order. The sorted unwind
>   * tables can be chained with a singly-linked list as they become full.
>   * Consecutive chained tables are also in sorted text address order.
>   *
>   * Note: if there is an eventual use-case for unsorted jit unwind table,
>   * this would be introduced as a new "code option".
>   */
> 
> struct code_jit_info {
>      __u64 text_start;      /* text_start >= addr */
>      __u64 text_end;        /* addr < text_end */
>      __u64 unwind_head;     /* struct code_jit_unwind_table * */
> };
> 
> struct code_jit_unwind_fre {
>      /*
>       * Contains info similar to sframe, allowing unwind for a given

Hmm, why not just the sframe?
(Is there any library to generate sframe online for JIT?)

Thank you,

>       * code address range.
>       */
>      __u32 size;
>      __u32 ip_off;  /* offset from text_start */
>      __s32 cfa_off;
>      __s32 ra_off;
>      __s32 fp_off;
>      __u8 info;
> };
> 
> struct code_jit_unwind_table {
>      __u64 reserved_len;
>      __u64 used_len; /*
>                       * Incremented by userspace (store-release), read by
>                       * the kernel (load-acquire).
>                       */
>      __u64 next;     /* Chain with next struct code_jit_unwind_table. */
>      struct code_jit_unwind_fre fre[];
> };
> 
> /* if (@option == CODE_UNREGISTER) */
> 
> void *info
> 
> * arg2: size_t info_size
> 
> /*
>   * Size of @info structure, allowing extensibility. See
>   * copy_struct_from_user().
>   */
> 
> * arg3: unsigned int flags (0)
> 
> /* Flags for extensibility. */
> 
> Your feedback is welcome,
> 
> Thanks,
> 
> Mathieu
> 
> [1] https://babeltrace.org/docs/v2.0/man7/babeltrace2-filter.lttng-utils.debug-info.7/
> 
> -- 
> Mathieu Desnoyers
> EfficiOS Inc.
> https://www.efficios.com
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>