[Qemu-devel] [RFC] configure script mistakenly detects static libraries

[Qemu-devel] [RFC] configure script mistakenly detects static libraries

[Murilo Opsfelder Araujo]

Hi, everyone!

I'm facing an issue on how configure script detects static libraries and
would like to hear from community to find a common ground on how to
possibly fix it.

Throughout configure, we use pkg-config command to verify if a library
is installed so qemu can be linked to it. This works fine when linking
qemu dynamically. However, configuring qemu with --static can mistakenly
detect a library that is actually not present on the system.

For example, on Ubuntu Xenial, libcacard-dev package provides only
libcacard.so (not libcacard.a) and pkg-config reports success in both
cases:

    $ pkg-config libcacard
    $ echo $?
    0

    $ pkg-config --static libcacard
    $ echo $?
    0

Since we use `pkg-config libcacard` to set smartcard=yes, this
mistakenly enables smartcard feature. This is acceptable with dynamic
linkage, but can be an issue with static linkage, where libcacard.a
doesn't exist on the system, resulting on a build error:

    $ ./configure --target-list=ppc64-softmmu --static && make -j$(nproc)
    [...]
    /usr/bin/ld: cannot find -lcacard

A workaround can be specifying --disable-<feature> for all missing
libraries.

One possible solution would be having a function, e.g. check_pkg_config,
that writes a C skeleton and tries to compile it using the desired
library, given as function parameter. Thus, compile_prog would return
success if library actually exists on the system. For example:

    check_pkg_config() {
        local_pkg_name=$1
        local_cflags=$($pkg_config --cflags $local_pkg_name)
        local_ldflags=$($pkg_config --libs $local_pkg_name)
        write_c_skeleton
        compile_prog "$local_cflags" "$local_ldflags"
    }

That would work for both dynamic and static linkages. Then we could just
replace `$pkg_config <package>` by `check_pkg_config <package>` in the
configure script.

My approach might not be the best and only one, so I'd like to hear from
you what would be a reasoanble solution to fix this situation.

Cheers
Murilo

Re: [Qemu-devel] [RFC] configure script mistakenly detects static libraries

[Peter Maydell]

On 23 April 2018 at 17:10, Murilo Opsfelder Araujo
<muriloo@linux.ibm.com> wrote:
> Hi, everyone!
>
> I'm facing an issue on how configure script detects static libraries and
> would like to hear from community to find a common ground on how to
> possibly fix it.
>
> Throughout configure, we use pkg-config command to verify if a library
> is installed so qemu can be linked to it. This works fine when linking
> qemu dynamically. However, configuring qemu with --static can mistakenly
> detect a library that is actually not present on the system.
>
> For example, on Ubuntu Xenial, libcacard-dev package provides only
> libcacard.so (not libcacard.a) and pkg-config reports success in both
> cases:
>
>     $ pkg-config libcacard
>     $ echo $?
>     0
>
>     $ pkg-config --static libcacard
>     $ echo $?
>     0
>
> Since we use `pkg-config libcacard` to set smartcard=yes, this
> mistakenly enables smartcard feature. This is acceptable with dynamic
> linkage, but can be an issue with static linkage, where libcacard.a
> doesn't exist on the system, resulting on a build error:

This seems to me to be an error in your distro's pkg-config information.
If static linking against libcacard doesn't work, then
"pkg-config --static libcacard" should fail.

Unfortunately IME the static linking support in distro-suppled
pkgconfig files is rarely tested, so it's not uncommon for it to
be broken.

>From an upstream QEMU point of view, we primarily support --static
for the benefit of the linux-user binaries, not for system emulation.
So we care more if a "configure --disable-system --disable-tools --static"
build doesn't work, than if the problem is only with features used
by the system emulator binaries.

thanks
-- PMM

Re: [Qemu-devel] [RFC] configure script mistakenly detects static libraries

[Daniel P. Berrangé]

On Mon, Apr 23, 2018 at 05:16:02PM +0100, Peter Maydell wrote:
> On 23 April 2018 at 17:10, Murilo Opsfelder Araujo
> <muriloo@linux.ibm.com> wrote:
> > Hi, everyone!
> >
> > I'm facing an issue on how configure script detects static libraries and
> > would like to hear from community to find a common ground on how to
> > possibly fix it.
> >
> > Throughout configure, we use pkg-config command to verify if a library
> > is installed so qemu can be linked to it. This works fine when linking
> > qemu dynamically. However, configuring qemu with --static can mistakenly
> > detect a library that is actually not present on the system.
> >
> > For example, on Ubuntu Xenial, libcacard-dev package provides only
> > libcacard.so (not libcacard.a) and pkg-config reports success in both
> > cases:
> >
> >     $ pkg-config libcacard
> >     $ echo $?
> >     0
> >
> >     $ pkg-config --static libcacard
> >     $ echo $?
> >     0
> >
> > Since we use `pkg-config libcacard` to set smartcard=yes, this
> > mistakenly enables smartcard feature. This is acceptable with dynamic
> > linkage, but can be an issue with static linkage, where libcacard.a
> > doesn't exist on the system, resulting on a build error:
> 
> This seems to me to be an error in your distro's pkg-config information.
> If static linking against libcacard doesn't work, then
> "pkg-config --static libcacard" should fail.

IIUC, --static isn't actually proving that a static version of a library
exists. Rather it just changes the logic for figuring out the dependancy
graph & compiler/linker flags. So --static will always work fine even if
only the .so is present, and no .a file exists.

Similarly pkg-config without --static doesn't actually guarantee that a
shared .so exists either - just tells you the flags you would need if
it did exist.

This works fine if you always build & ship static & shared versions of
every library, but distros often ditch static builds of all but a
handful of libraries, because of the ripple effect static linking
has when security updates are issued.

> Unfortunately IME the static linking support in distro-suppled
> pkgconfig files is rarely tested, so it's not uncommon for it to
> be broken.
> 
> From an upstream QEMU point of view, we primarily support --static
> for the benefit of the linux-user binaries, not for system emulation.
> So we care more if a "configure --disable-system --disable-tools --static"
> build doesn't work, than if the problem is only with features used
> by the system emulator binaries.

That reduces the 3rd party deps to essentially just your C library and
glib2, which makes it more tractable from a security update POV. I would
certainly not wish to see static linking of a system emulator which pulls
in 150+ 3rd party libraries on my system !

Regards,
Daniel
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Re: [Qemu-devel] [RFC] configure script mistakenly detects static libraries

[Murilo Opsfelder Araujo]

On 04/23/2018 01:16 PM, Peter Maydell wrote:
> On 23 April 2018 at 17:10, Murilo Opsfelder Araujo
> <muriloo@linux.ibm.com> wrote:
>> Hi, everyone!
>>
>> I'm facing an issue on how configure script detects static libraries and
>> would like to hear from community to find a common ground on how to
>> possibly fix it.
>>
>> Throughout configure, we use pkg-config command to verify if a library
>> is installed so qemu can be linked to it. This works fine when linking
>> qemu dynamically. However, configuring qemu with --static can mistakenly
>> detect a library that is actually not present on the system.
>>
>> For example, on Ubuntu Xenial, libcacard-dev package provides only
>> libcacard.so (not libcacard.a) and pkg-config reports success in both
>> cases:
>>
>>     $ pkg-config libcacard
>>     $ echo $?
>>     0
>>
>>     $ pkg-config --static libcacard
>>     $ echo $?
>>     0
>>
>> Since we use `pkg-config libcacard` to set smartcard=yes, this
>> mistakenly enables smartcard feature. This is acceptable with dynamic
>> linkage, but can be an issue with static linkage, where libcacard.a
>> doesn't exist on the system, resulting on a build error:
> 
> This seems to me to be an error in your distro's pkg-config information.
> If static linking against libcacard doesn't work, then
> "pkg-config --static libcacard" should fail.

I see in man(1) pkg-config that --static is merely used to compute
deeper dependency graph intended for static linking.

Since <package>.pc is present, pkg-config assumes the library exists and
--static is merely informational.

Do you think this should be reported to pkg-config project?

> Unfortunately IME the static linking support in distro-suppled
> pkgconfig files is rarely tested, so it's not uncommon for it to
> be broken.
> 
> From an upstream QEMU point of view, we primarily support --static
> for the benefit of the linux-user binaries, not for system emulation.
> So we care more if a "configure --disable-system --disable-tools --static"
> build doesn't work, than if the problem is only with features used
> by the system emulator binaries.

In this case, shouldn't we update configure script to emit a warning
message to user saying it's running a non-supported scenario when
--static is given without --disable-system and --disable-tools?

> thanks
> -- PMM
> 

Cheers
Murilo