72 lines
2.9 KiB
Diff
72 lines
2.9 KiB
Diff
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From e72c9d7ead60e3317bd6d1fade995c07021c947b Mon Sep 17 00:00:00 2001
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From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
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Date: Thu, 7 May 2015 13:25:04 -0400
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Subject: [PATCH] Fix: building probe providers with C++ compiler
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Robert Daniels wrote:
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> > I'm attempting to use lttng userspace tracing with a C++ application
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> > on an ARM platform. I'm using GCC 4.8.4 on Linux 3.14 with the 2.6
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> > release of lttng. I've compiled lttng-modules, lttng-ust, and
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> > lttng-tools and have been able to get a simple test working with C
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> > code. When I attempt to run the hello.cxx test on my target it will
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> > segfault.
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>
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>
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> I spent a little time digging into this issue and finally discovered the
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> cause of my segfault with ARM C++ tracepoints.
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>
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> There is a struct called 'lttng_event' in ust-events.h which contains an
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> empty union 'u'. This was the cause of my issue. Under C, this empty union
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> compiles to a zero byte member while under C++ it compiles to a one byte
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> member, and in my case was four-byte aligned which caused my C++ code to
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> have the 'cds_list_head node' offset incorrectly by four bytes. This lead
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> to an incorrect linked list structure which caused my issue.
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>
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> Since this union is empty, I simply removed it from the struct and everything
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> worked correctly.
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>
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> I don't know the history or purpose behind this empty union so I'd like to
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> know if this is a safe fix. If it is I can submit a patch with the union
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> removed.
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That's a very nice catch!
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We do not support building tracepoint probe provider with
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g++ yet, as stated in lttng-ust(3):
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"- Note for C++ support: although an application instrumented with
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tracepoints can be compiled with g++, tracepoint probes should be
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compiled with gcc (only tested with gcc so far)."
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However, if it works fine with this fix, then I'm tempted to take it,
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especially because removing the empty union does not appear to affect
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the layout of struct lttng_event as seen from liblttng-ust, which must
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be compiled with a C compiler, and from probe providers compiled with
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a C compiler. So all we are changing is the layout of a probe provider
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compiled with a C++ compiler, which is anyway buggy at the moment,
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because it is not compatible with the layout expected by liblttng-ust
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compiled with a C compiler.
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Reported-by: Robert Daniels <robert.daniels@vantagecontrols.com>
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Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
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---
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include/lttng/ust-events.h | 2 --
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1 file changed, 2 deletions(-)
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diff --git a/usr/include/lttng/ust-events.h b/usr/include/lttng/ust-events.h
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index 328a875..3d7a274 100644
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--- a/usr/include/lttng/ust-events.h
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+++ b/usr/include/lttng/ust-events.h
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@@ -407,8 +407,6 @@ struct lttng_event {
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void *_deprecated1;
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struct lttng_ctx *ctx;
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enum lttng_ust_instrumentation instrumentation;
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- union {
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- } u;
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struct cds_list_head node; /* Event list in session */
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struct cds_list_head _deprecated2;
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void *_deprecated3;
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--
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2.7.4
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