On Mon, 21 Jul 2025 11:20:34 -0400
Mathieu Desnoyers <mathieu.desnoy...@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.
Hmm, I guess I'm OK with that name. I can't really think of anything that
would be better. But kernel developers are notorious for sucking at coming
up with decent names ;-)
>
> 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.
> 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.
Honestly, I'm not sure it needs to be an ELF file. Just a file that has an
sframe section in it.
>
> sys_codectl(2)
> =================
>
> * arg0: unsigned int @option:
>
> /* Additional labels can be added to enum code_opt, for extensibility. */
>
> enum code_opt {
> CODE_REGISTER_ELF,
Perhaps the above should be: CODE_REGISTER_SFRAME,
as currently SFrame is read only via files.
> CODE_REGISTER_JIT,
From our other conversations, JIT will likely be a completely different
format than SFRAME, so calling it just JIT should be fine.
> CODE_UNREGISTER,
I wonder if this should be the first enum. That is, "0" is to unregister.
That way, all non-zero options will be for what is being registered, and
"0" is for unregistering any of them.
> };
>
> * 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;
Perhaps:
__u64 file_start;
__u64 file_end;
?
And call it "struct code_sframe_info"
> __u64 text_start;
> __u64 text_end;
> __u64 sframe_start;
> __u64 sframe_end;
What is the above "sframe" for?
> __u64 pathname; /* char *, NULL if unavailable. */
>
> __u64 build_id; /* char *, NULL if unavailable. */
> __u64 debug_link_pathname; /* char *, NULL if unavailable. */
Maybe just list the above three as "optional" ?
It may be available, but the implementer just doesn't want to implement it.
> __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
> * 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[];
> };
I wonder if we should avoid the "jit" portion completely for now until we
know what exactly we need.
Thanks,
-- Steve
>
> /* 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/
>
