Fix: liblttng-ctl: ABI-breaking size change of lttng_session struct

abidiff reports that the size of struct lttng_session has changed
since 2.10:

  [C]'function int lttng_list_sessions(lttng_session**)' at lttng-ctl.c:2065:1 has some indirect sub-type changes:
    parameter 1 of type 'lttng_session**' has sub-type changes:
      in pointed to type 'lttng_session*':
        in pointed to type 'struct lttng_session' at session.h:38:1:
          type size changed from 35008 to 35072 (in bits)
          1 data member deletion:
            'char lttng_session::padding[12]', at offset 34912 (in bits) at session.h:50:1

          1 data member insertion:
            'union {char padding[12]; void* ptr;} lttng_session::extended', at offset 34944 (in bits) at session.h:57:1

The offset after the 'live_timer_interval' field is aligned on 4
bytes, but not on 8 bytes. This causes some compilers (such as gcc and
clang) to align the following 'extended' union on 8 bytes, making the
overall structure larger.

To preserve the size of 'struct lttng_session', four bytes of padding
are added after 'live_timer_interval', resulting in an aligned offset
for both bitnesses.

The 'extended' union's padding is reduced from 12 to 8 bytes,
essentially ensuring that 'ptr' always occupies 8 bytes, even on
32-bit builds.

Tested on clang and gcc for x64, x86, PPC32, PPC64, ARM, ARM64,
AVR, MIPS, MIPS64, and MSVC (32-bit and 64-bit).

Reviewed-by: Michael Jeanson <michael.jeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>

diff --git a/include/lttng/session.h b/include/lttng/session.h
index 395df32450889d806aa0efd805a7a2cd67f4c219..7e328b625f29ffcef9faf6fbcdc1172a83f45805 100644 (file)
--- a/include/lttng/session.h
+++ b/include/lttng/session.h
@@ -34,7 +34,7 @@ struct lttng_destruction_handle;
  *
  * The structures should be initialized to zero before use.
  */
-#define LTTNG_SESSION_PADDING1             12
+#define LTTNG_SESSION_PADDING1             8
 struct lttng_session {
        char name[LTTNG_NAME_MAX];
        /*
@@ -51,7 +51,29 @@ struct lttng_session {
        uint32_t snapshot_mode;
        unsigned int live_timer_interval;       /* usec */
 
+       /*
+        * End of public attributes.
+        * The remaining fields are used to deal with ABI management concerns.
+        */
+
+       /*
+        * 32-bit architectures are already naturally aligned on 4 bytes after
+        * 'live_timer_interval'. However, the offset does not result in a
+        * natural alignment on 64-bit architectures. Adding 4 bytes of
+        * padding here results in an aligned offset after 'alignement_padding'
+        * for both bitnesses.
+        *
+        * This was added since not all compilers appear to align unions in the
+        * same way. Some (e.g. MSVC) do not seem to impose an alignement
+        * constraint while others (e.g. gcc, clang, icc) seem to align it to
+        * ensure 'ptr' is naturally aligned.
+        */
+       char alignment_padding[4];
        union {
+               /*
+                * Ensure the 'extended' union has the same size for both
+                * 32-bit and 64-bit builds.
+                */
                char padding[LTTNG_SESSION_PADDING1];
                void *ptr;
        } extended;