Re: [PATCH v2] trace: use STAP_SDT_V2 to work around symbol visibility

[Miroslav Rezanina]

----- Original Message -----
> From: "Stefan Hajnoczi" <stefanha@redhat.com>
> To: qemu-devel@nongnu.org
> Cc: "Peter Maydell" <peter.maydell@linaro.org>, "Stefan Hajnoczi" 
> <stefanha@redhat.com>, "Daniel P . Berrangé"
> <berrange@redhat.com>, wcohen@redhat.com, fche@redhat.com, kraxel@redhat.com, 
> rjones@redhat.com,
> mrezanin@redhat.com, ddepaula@redhat.com
> Sent: Thursday, November 19, 2020 12:27:04 PM
> Subject: [PATCH v2] trace: use STAP_SDT_V2 to work around symbol visibility
> 
> QEMU binaries no longer launch successfully with recent SystemTap
> releases. This is because modular QEMU builds link the sdt semaphores
> into the main binary instead of into the shared objects where they are
> used. The symbol visibility of semaphores is 'hidden' and the dynamic
> linker prints an error during module loading:
> 
>   $ ./configure --enable-trace-backends=dtrace --enable-modules ...
>   ...
>   Failed to open module:
>   /builddir/build/BUILD/qemu-4.2.0/s390x-softmmu/../block-curl.so: undefined
>   symbol: qemu_curl_close_semaphore
> 
> The long-term solution is to generate per-module dtrace .o files and
> link them into the module instead of the main binary.
> 
> In the short term we can define STAP_SDT_V2 so dtrace(1) produces a .o
> file with 'default' symbol visibility instead of 'hidden'. This
> workaround is small and easier to merge for QEMU 5.2.
> 

Thanks for this fix.

Reviewed-by: Miroslav Rezanina <mrezanin@redhat.com>

> Cc: Daniel P. Berrangé <berrange@redhat.com>
> Cc: wcohen@redhat.com
> Cc: fche@redhat.com
> Cc: kraxel@redhat.com
> Cc: rjones@redhat.com
> Cc: mrezanin@redhat.com
> Cc: ddepaula@redhat.com
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
> v2:
>  * Define STAP_SDT_V2 everywhere [danpb]
> ---
>  configure         | 1 +
>  trace/meson.build | 4 ++--
>  2 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/configure b/configure
> index 714e75b5d8..5d91d49c7b 100755
> --- a/configure
> +++ b/configure
> @@ -4832,6 +4832,7 @@ if have_backend "dtrace"; then
>    trace_backend_stap="no"
>    if has 'stap' ; then
>      trace_backend_stap="yes"
> +    QEMU_CFLAGS="$QEMU_CFLAGS -DSTAP_SDT_V2"
>    fi
>  fi
>  
> diff --git a/trace/meson.build b/trace/meson.build
> index d5fc45c628..843ea14495 100644
> --- a/trace/meson.build
> +++ b/trace/meson.build
> @@ -38,13 +38,13 @@ foreach dir : [ '.' ] + trace_events_subdirs
>      trace_dtrace_h = custom_target(fmt.format('trace-dtrace', 'h'),
>                                     output: fmt.format('trace-dtrace', 'h'),
>                                     input: trace_dtrace,
> -                                   command: [ 'dtrace', '-o', '@OUTPUT@',
> '-h', '-s', '@INPUT@' ])
> +                                   command: [ 'dtrace', '-DSTAP_SDT_V2',
> '-o', '@OUTPUT@', '-h', '-s', '@INPUT@' ])
>      trace_ss.add(trace_dtrace_h)
>      if host_machine.system() != 'darwin'
>        trace_dtrace_o = custom_target(fmt.format('trace-dtrace', 'o'),
>                                       output: fmt.format('trace-dtrace',
>                                       'o'),
>                                       input: trace_dtrace,
> -                                     command: [ 'dtrace', '-o', '@OUTPUT@',
> '-G', '-s', '@INPUT@' ])
> +                                     command: [ 'dtrace', '-DSTAP_SDT_V2',
> '-o', '@OUTPUT@', '-G', '-s', '@INPUT@' ])
>        trace_ss.add(trace_dtrace_o)
>      endif
>  
> --
> 2.28.0
> 
> 

-- 
Miroslav Rezanina
Software Engineer - Virtualization Team Maintainer