Re: (interposers): How to workaround the "strong symbols" problem?

On 2025-01-17, Kenny McCormack <gazelle@shell.xmission.com> wrote:

In article <vmeibp$33sam$1@news.xmission.com>,
Kenny McCormack <gazelle@shell.xmission.com> wrote:

Context is Linux (and only Linux).
>
Over the years, I have written many "interposers" - that is, a shared
library loaded with LD_PRELOAD that hooks some system or library call
(e.g., "read").  The interposer usually ends up calling the "real"
function, then doing something special either before or after the call.

>
Also, wanted to state that the function I am trying to hook is "openat".
>
"open" I can hook, but "openat" doesn't work.  I think most programs just
call "open", which eventually ends up calling "openat", so that (hooking
"open") works.  But some programs call "openat" directly, and that doesn't
work.
>
And, also, just to clarify, the whole point of this is to modify the
behavior of a program without the bother of re-compiling it.

In one installation where glibc is 2.31 from 2021, I get this:

$ nm /lib64/libc.so.6  | grep '1444b0'
00000000001444b0 t __GI___openat
00000000001444b0 t __GI___openat64
00000000001444b0 t __libc_openat64
00000000001444b0 t __openat
00000000001444b0 W openat
00000000001444b0 t __openat64
00000000001444b0 W openat64

Where 1444b0 is the address I discovered by another grep, and
then used it to see what all the aliases are. The openat and
openat64 symbols are W (weak).

If you do the same exercise, what do you see?

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