[PATCH v3] trace: use STAP_SDT_V2 to work around symbol visibility

Stefan Hajnoczi posted 1 patch 4 days, 16 hours ago
Test checkpatch passed
Patches applied successfully (tree, apply log)
git fetch https://github.com/patchew-project/qemu tags/patchew/20201119141457.844452-1-stefanha@redhat.com
configure         | 7 +++++++
trace/meson.build | 4 ++--
2 files changed, 9 insertions(+), 2 deletions(-)

[PATCH v3] trace: use STAP_SDT_V2 to work around symbol visibility

Posted by Stefan Hajnoczi 4 days, 16 hours ago
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 and downstream
backports.

Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1898700
Cc: wcohen@redhat.com
Cc: fche@redhat.com
Cc: kraxel@redhat.com
Cc: rjones@redhat.com
Cc: ddepaula@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Miroslav Rezanina <mrezanin@redhat.com>
---
v3:
 * Tweak commit description for clarity [danpb]
 * Add Buglink tag
 * Add explanation comment into ./configure [Philippe]
 * Add Reviewed-by tags
v2:
 * Define STAP_SDT_V2 everywhere [danpb]
---
 configure         | 7 +++++++
 trace/meson.build | 4 ++--
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/configure b/configure
index 714e75b5d8..796cec14de 100755
--- a/configure
+++ b/configure
@@ -4832,6 +4832,13 @@ if have_backend "dtrace"; then
   trace_backend_stap="no"
   if has 'stap' ; then
     trace_backend_stap="yes"
+
+    # Workaround to avoid dtrace(1) producing a file with 'hidden' symbol
+    # visibility. Define STAP_SDT_V2 to produce 'default' symbol visibility
+    # instead. QEMU --enable-modules depends on this because the SystemTap
+    # semaphores are linked into the main binary and not the module's shared
+    # object.
+    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

Re: [PATCH v3] trace: use STAP_SDT_V2 to work around symbol visibility

Posted by Philippe Mathieu-Daudé 4 days, 15 hours ago
On 11/19/20 3:14 PM, 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 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 and downstream
> backports.
> 
> Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1898700
> Cc: wcohen@redhat.com
> Cc: fche@redhat.com
> Cc: kraxel@redhat.com
> Cc: rjones@redhat.com
> Cc: ddepaula@redhat.com
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
> Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> Reviewed-by: Miroslav Rezanina <mrezanin@redhat.com>
> ---
> v3:
>  * Tweak commit description for clarity [danpb]
>  * Add Buglink tag
>  * Add explanation comment into ./configure [Philippe]
>  * Add Reviewed-by tags
> v2:
>  * Define STAP_SDT_V2 everywhere [danpb]
> ---
>  configure         | 7 +++++++
>  trace/meson.build | 4 ++--
>  2 files changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/configure b/configure
> index 714e75b5d8..796cec14de 100755
> --- a/configure
> +++ b/configure
> @@ -4832,6 +4832,13 @@ if have_backend "dtrace"; then
>    trace_backend_stap="no"
>    if has 'stap' ; then
>      trace_backend_stap="yes"
> +
> +    # Workaround to avoid dtrace(1) producing a file with 'hidden' symbol
> +    # visibility. Define STAP_SDT_V2 to produce 'default' symbol visibility
> +    # instead. QEMU --enable-modules depends on this because the SystemTap
> +    # semaphores are linked into the main binary and not the module's shared
> +    # object.

Thanks!

> +    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
>  
>