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

[Daniel P . Berrangé]

On Wed, Nov 18, 2020 at 05:48:09PM +0000, Stefan Hajnoczi wrote:
> 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 /usr/bin/dtrace produces
> an .o file with 'default' symbol visibility instead of 'hidden'. This
> workaround is small and easier to merge for QEMU 5.2.
> 
> 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>
> ---
>  trace/meson.build | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/trace/meson.build b/trace/meson.build
> index d5fc45c628..52be7c5b2c 100644
> --- a/trace/meson.build
> +++ b/trace/meson.build
> @@ -44,7 +44,7 @@ foreach dir : [ '.' ] + trace_events_subdirs
>        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@' ])

I'm a little concerned that we're not also setting this macro before
including the generated trace.h headers, because those headers do
check this STAP_SDT_V1 symbol.

Currently the generated headers have same code for V2 and V3 (the default),
so we won't break, but I'm concerned we could break if they introduce a
future V4 and that impacts the generated headers.

So I think the safe thing todo is set -DSTAP_SDT_V2 as a global compile
arg for QEMU too, so all trace.h files see the symbol that matches the
trace.o files


Regards,
Daniel
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