Re: [RFC PATCH] ftrace: Skip __fentry__ location of overridden weak functions

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



On 2024/6/7 23:02, Peter Zijlstra wrote:
On Fri, Jun 07, 2024 at 07:52:11PM +0800, Zheng Yejian wrote:
ftrace_location() was changed to not only return the __fentry__ location
when called for the __fentry__ location, but also when called for the
sym+0 location after commit aebfd12521d9 ("x86/ibt,ftrace: Search for
__fentry__ location"). That is, if sym+0 location is not __fentry__,
ftrace_location() would find one over the entire size of the sym.

However, there is case that more than one __fentry__ exist in the sym
range (described below) and ftrace_location() would find wrong __fentry__
location by binary searching, which would cause its users like livepatch/
kprobe/bpf to not work properly on this sym!

The case is that, based on current compiler behavior, suppose:
  - function A is followed by weak function B1 in same binary file;
  - weak function B1 is overridden by function B2;
Then in the final binary file:
  - symbol B1 will be removed from symbol table while its instructions are
    not removed;
  - __fentry__ of B1 will be still in __mcount_loc table;
  - function size of A is computed by substracting the symbol address of
    A from its next symbol address (see kallsyms_lookup_size_offset()),
    but because symbol info of B1 is removed, the next symbol of A is
    originally the next symbol of B1. See following example, function
    sizeof A will be (symbol_address_C - symbol_address_A):

      symbol_address_A
      symbol_address_B1 (Not in symbol table)
      symbol_address_C

The weak function issue has been discovered in commit b39181f7c690
("ftrace: Add FTRACE_MCOUNT_MAX_OFFSET to avoid adding weak function")
but it didn't resolve the issue in ftrace_location().

There may be following resolutions:

Oh gawd, sodding weak functions again.

I would suggest changing scipts/kallsyms.c to emit readily identifiable
symbol names for all the weak junk, eg:

   __weak_junk_NNNNN


Sorry for the late reply, I just had a long noon holiday :>

scripts/kallsyms.c is compiled and used to handle symbols in vmlinux.o
or vmlinux.a, see kallsyms_step() in scripts/link-vmlinux.sh, those
overridden weak symbols has been removed from symbol table of vmlinux.o
or vmlinux.a. But we can found those symbols from original xx/xx.o file,
for example, the weak free_initmem() in in init/main.c is overridden,
its symbol is not in vmlinx but is still in init/main.o .

How about traversing all origin xx/xx.o and finding all weak junk symbols ?

That instantly fixes the immediate problem and Steve's horrid hack can
go away.


Yes, this can be done in same patch series.

Additionally, I would add a boot up pass that would INT3 fill all such
functions and remove/invalidate all
static_call/static_jump/fentry/alternative entry that is inside of them.




--

Thanks,
Zheng Yejian




[Index of Archives]     [Linux USB Devel]     [Video for Linux]     [Linux Audio Users]     [Yosemite News]     [Linux Kernel]     [Linux SCSI]     [Big List of Linux Books]

  Powered by Linux