AW: dlopen: Segfault due to overwriting .so file after it was loaded and loading it again

[Wendeborn, Jonathan]

Hi Florian,

Thank you, that sounds reasonable. Indeed the library is marked NODELETE due to unique symbols. Is it forbidden in general to write executables which are loaded?

I attached the outputs of strace and LD_DEBUG=all if you're interested.

Thanks!
Jonathan 

-----Ursprüngliche Nachricht-----
Von: Florian Weimer <fw@deneb.enyo.de> 
Gesendet: Freitag, 20. November 2020 12:33
An: Wendeborn, Jonathan via Libc-help <libc-help@sourceware.org>
Cc: Wendeborn, Jonathan <Jonathan.Wendeborn@bruker.com>
Betreff: Re: dlopen: Segfault due to overwriting .so file after it was loaded and loading it again

**EXTERNAL EMAIL**

* Jonathan via Libc-help Wendeborn:

> The destructor ~shared_library() calls dlclose(), but I suspect the 
> library stays loaded. Overwriting the file creates a new file node and 
> my program wants to load the same library again (at the same location 
> but with a different file node/handle).

This crash happens if the file is truncated on disk and rewritten.
All mapped data is reset to zero if that happens and relocations are gone.  The kernel does that, there is nothing that glibc can do about it (Linux does not support MAP_COPY).

So you have two issues here: The library stays loaded (maybe it is marked as NODELETE, LD_DEBUG=all output logs that in recent glibc versions), and the file is rewritten in place (and not a “new file node” is created).
-------------- next part --------------
A non-text attachment was scrubbed...
Name: output.tar.gz
Type: application/x-gzip
Size: 253917 bytes
Desc: output.tar.gz
URL: <https://sourceware.org/pipermail/libc-help/attachments/20201120/1b2f4d0e/attachment-0001.bin>