On Fri, Apr 4, 2025, at 5:04 PM, Serhei Makarov wrote:
> Changes for v2:
>
> - Add locking for elftab. This is needed in addition to the
> intrinsic locking in dynamicsizehash_concurrent to avoid
> having cache_elf expose an incomplete dwfltracker_elf_info*
> entry to other threads while its data is being populated /
> replaced.
>
> - Tidy dwfl_process_tracker_find_elf.c into the main find_elf
> callback and two functions to consider (in future) making into
> a public api for custom cached callbacks.
Note: going to apply one nontrivial change to this patch -- taking dev/ino into
account in the caching key,
to allow caching modules in container filesystems (which Sysprof accesses via
/proc/PID/root).
The "/proc/PID/root/MODULE" path is unique per-process, so we need to cache by
"MODULE+dev+ino"
in order to catch repeated instantiations of a module from a container image.
The concept is fairly clear, but I'm muddled on how to expose it in the public
API.
Option 1: expose two find_cached_elf functions.
// normal use case
module_name = "/usr/lib/whatever.so";
dwfl_process_tracker_find_cached_elf (tracker, module_name, &file_name, &elf,
&fd);
/* This will access the file at module_name */
// use case for executable in container
module_name = "/usr/lib/whatever.so";
file_name = "/proc/PID/root/usr/lib/whatever.so";
struct stat sb; stat (file_name, &sb);
dwfl_process_tracker_check_cached_elf_path (tracker, module_name,
sb.st_dev, sb.st_ino, file_name, &elf, &fd);
/* This will access the file at file_name
Option 2: expose one function, treat file_name and fd as optional input
parameters.
// normal use case
module_name = "/usr/lib/whatever.so"; /* name of module */
file_name = "/proc/PID/root/usr/lib/whatever.so"; /* actual location of module
*/
dwfl_process_tracker_find_cached_elf (tracker, module_name, &file_name, &elf,
&fd);
/* Since file_name is specified, the dwfl_process_tracker
will access the module at file_name, and cache by
module_name + dev + ino. */
Either way, Sysprof can include its own version of the
dwfl_process_tracker_find_elf()
function (sysprof_live_process_find_elf) which calls these as appropriate.
I'm leaning towards Option 2 but wanted to present both for consideration.
