Re: get_relocated_path: the configured paths are not looked for?

[Akihiko Odaki <akihiko.odaki@gmail.com>]

On 2023/04/23 19:28, Michael Tokarev wrote:
> 20.04.2023 08:29, Akihiko Odaki wrote:
>> On 2023/04/19 16:32, Michael Tokarev wrote:
>>> Hello!
>>>
>>> Today I discovered an interesting issue here: I copied a 
>>> system-installed
>>> binary into another directory, in order to debug an unrelated issue. 
>>> Just
>>> to discover it does not work, being unable to find any modules or data
>>> files.
>>>
>>> Here's how the strace of typical qemu-system-i386 run looks like (the
>>> relevant parts only):
>>>
>>> access("/tmp/qemu-bundle", R_OK) = -1 ENOENT (No such file or directory)
>>> access("/tmp/b/../lib/x86_64-linux-gnu/qemu/accel-tcg-i386.so", F_OK) 
>>> = -1 ENOENT (No such file or directory)
>>> access("/var/run/qemu/Debian_1_8.0~rc4+dfsg-3/accel-tcg-i386.so", 
>>> F_OK) = -1 ENOENT (No such file or directory)
>>>
>>> (the executable in this case is in /tmp, obviously).  And it fails with
>>> error "fatal: could not load module for type 'tcg-accel-ops'".
>>>
>>> This is despite the fact that qemu has been configured with proper 
>>> --libdir
>>> and other --foodir to point to actual dirs such as /usr/lib 
>>> /usr/share etc.
>>
>> Some files in QEMU installation is closely coupled with the binary so 
>> it does not make much sense to copy only the executable to another 
>> directory. You need to copy all of the files QEMU owns to relocate a 
>> QEMU installation. QEMU uses relative paths to find such relocated files.
>>
>> That said, it is indeed confusing that QEMU uses relative paths even 
>> if you specify an absolute path for --libdir. I prefer to require to 
>> specify relative paths for --libdir and other options to make a QEMU 
>> installation relocatable, but I didn't dare making such a breaking 
>> change.
> 
> Well, quite often it makes little sense still.
> 
> For example, in debian we had to (temporarily) move qemu-system-i386 from
> /usr/bin/ to /usr/libexec/, replacing the /usr/bin/ one with a shell 
> wrapper
> (to decouple xen hvm build out of main qemu build). It was just by a chance
> the directory nesting is the same still, - I wanted to move it to 
> /usr/lib/qemu/
> instead.  But at least this one still works.  Ditto for the actual xen 
> version,
> the binary is in /usr/libexec/qemu-system-i386-xen now, I was about to 
> move it
> to /usr/libexec/xen/qemu-system-i386 instead.
> 
> I see absolutely no reason for the binary to look into 
> /usr/libexec/share/qemu/bios.bin
> if it is told to get all data files from /usr/share/qemu/.

What about specifying --bindir=/usr/libexec/xen?

> 
> Quite often I debug issues and have to compare "old" qemu with current one
> (exactly like I did in this case), extracting the old binary into /tmp/.
> And wonder why it breaks in other ways.
> 
> And I still don't see how this turning absolute paths into relative 
> (behind the
> scenes) can be useful. If we put qemu into /usr/local/bin for example, it
> can be configured to look for data files in /usr/local/share just fine,
> there's no need for this "magic". On the other hand, if we really want
> to use a different data dir, we can pass a right -L option.

As you say, data files are not tightly coupled with QEMU version and it 
is quite possible to run QEMU with data files from another QEMU version.

However it's not true for all files QEMU installs. Especially module 
files like accel-tcg-i386.so are tightly coupled with the executable and 
QEMU is not expected to work if the exact modules which came with the 
executable are not provided.

> 
> I think I'll just remove this (very questionable) behavior at least from 
> the
> Debian package.  I already patched out the previous version of this, when
> it looked at ${exe_path}/../pc-bios - this at least made sense when we
> needed to find firmware files for just-built qemu, in the source dir. But
> it makes no sense (in my opinion) to do that for the installed version.

It uses qemu-bundle directory for just-built QEMU so it does not matter 
for the scenario.

The scenario where this conversion for relative path makes sense is the 
case you need to compare the behavior of two packaged versions of QEMU, 
for example. In such case, you can extract each package into a different 
directory and run them. Each QEMU executable will use relative paths to 
find modules tied with it.

If you are removing this behavior from Debian package you may consult 
other package maintainers, especially GCC package maintainers. If I 
remember correctly, GCC also has similar logic to find executables like 
cc1 and shared objects like LTO plugins.

Regards,
Akihiko Odaki

> 
> Thanks,
> 
> /mjt