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

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

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.