Re: [RFC PATCH v1] tracing: Fix syscall tracepoint use-after-free

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2024-10-26 10:25, Mathieu Desnoyers wrote:
> On 2024-10-26 03:13, Steven Rostedt wrote:
>> On Fri, 25 Oct 2024 15:38:48 -0400
>> Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
>>
>>>> I'm curious if it might be better to add some field to struct
>>>> tracepoint like "sleepable" rather than adding a special case here
>>>> based on the name? Of course, if it's only ever going to be these
>>>> two cases then maybe adding a new field doesn't make sense.
>>>
>>> I know Steven is reluctant to bloat the tracepoint struct because there
>>> are lots of tracepoint instances (thousands). So for now I thought that
>>> just comparing the name would be a good start.
>>
>> You are correct. I really trying to keep the footprint of
>> tracepoints/events down.
>>
>>>
>>> We can eventually go a different route as well: introduce a section just
>>> to put the syscall tracepoints, and compare the struct tracepoint
>>> pointers to the section begin/end range. But it's rather complex
>>> for what should remain a simple fix.
>>
>> A separate section could work.
> 
> I have another approach to suggest: it shrinks the
> size of struct tracepoint from 80 bytes down to 72 bytes
> on x86-64, we don't have to do any section/linker
> script trickery, and it's extensible for future flags:
> 
> struct static_key {
>          int enabled;
>          void *p;
> };
> 
> struct static_key_false {
>          struct static_key key;
> };
> 
> struct static_call_key {
>          void *func;
>          void *p;
> };
> 
> struct tracepoint {
>          const char *name;               /* Tracepoint name */
>          struct static_key_false key;
>          struct static_call_key *static_call_key;
>          void *static_call_tramp;
>          void *iterator;
>          void *probestub;
>          void *funcs;
>          /* Flags. */
>          unsigned int regfunc:1,
>                       syscall:1;
> };
> 
> struct tracepoint_regfunc {
>          struct tracepoint tp;
>          int (*regfunc)(void);
>          void (*unregfunc)(void);
> };
> 
> Basically, a tracepoint with regfunc would define a
> struct tracepoint_regfunc rather than a struct tracepoint.
> So we remove both regfunc and unregfunc NULL pointers in
> the common case, which gives us plenty of room for flags.
> 
> When we want to access the regfunc/unregfunc from
> a struct tracepoint, we check the regfunc flag, and
> if set, we can use container_of() to get the struct
> tracepoint_regfunc.

Actually I can achieve the same space saving with fewer
changes like this:

struct tracepoint_ext {
     void *regfunc;
     void *unregfunc;
     /* Flags. */
     unsigned int syscall:1;
}

struct tracepoint {
         const char *name;               /* Tracepoint name */
         struct static_key_false key;
         struct static_call_key *static_call_key;
         void *static_call_tramp;
         void *iterator;
         void *probestub;
         void *funcs;
         struct tracepoint_ext *ext;
};

Thanks,

Mathieu

> 
> Thoughts ?
> 
> Thanks,
> 
> Mathieu
> 

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com