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Re: [PATCH] tile: Fix BZ #18508 (makecontext yield infinite backtrace)

Sorry, forgot to mark this patch as [COMMITTED].

This is the last change I have for the 2.22 release.

On 07/21/2015 12:13 PM, Chris Metcalf wrote:
It turns out tile suffered from the same problem as S390.  However,
disabling CFI information for the __startcontext on tile was not
sufficient to fix the problem; I think the backtracer will just
blindly try to follow the link register (lr) in that case.

Instead, the change adds a cfi_undefined directive for "lr"
and then arranges to call __startcontext directly when the new
context starts, rather than just synthesizing a return to it.
In addition to being a bit easier now to understand the control
flow, this also allows the cfi_undefined directive to be placed in
a way that causes it to be in force at the address that the "lr"
from the called function points to.
  ChangeLog                                  | 7 +++++++
  sysdeps/unix/sysv/linux/tile/makecontext.c | 8 ++++----
  sysdeps/unix/sysv/linux/tile/setcontext.S  | 2 ++
  3 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 490c0acf962f..d855f769e19e 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+2015-07-21  Chris Metcalf  <>
+	* sysdeps/unix/sysv/linux/tile/makecontext.c (__makecontext): Call
+	__startcontext to initialize the new context.
+	* sysdeps/unix/sysv/linux/tile/setcontext.S (__startcontext): Set
+	up CFI directive to forbid further backtracing.
  2015-07-21  Marko Myllynen  <>
* charmaps/ANSI_X3.110-1983: Remove obsolete repertoire map
diff --git a/sysdeps/unix/sysv/linux/tile/makecontext.c b/sysdeps/unix/sysv/linux/tile/makecontext.c
index b14b8d5cc91f..c77d00567296 100644
--- a/sysdeps/unix/sysv/linux/tile/makecontext.c
+++ b/sysdeps/unix/sysv/linux/tile/makecontext.c
@@ -52,14 +52,14 @@ __makecontext (ucontext_t *ucp, void (*func) (void), int argc, ...)
    va_end (ap);
- /* Pass (*func) to __startcontext in pc. */
-  ucp->uc_mcontext.pc = (long) func;
+  /* Start in the trampoline.  */
+  ucp->uc_mcontext.pc = (long) __startcontext;
/* Set stack pointer. */
    ucp->uc_mcontext.sp = (long) sp;
- /* Set the return address to trampoline. */
-  ucp-> = (long) __startcontext;
+  /* Pass FUNC to __startcontext in r31.  */
+  ucp->uc_mcontext.gregs[31] = (long) func;
/* Pass ucp->uc_link to __startcontext in r30. */
    ucp->uc_mcontext.gregs[30] = (long) ucp->uc_link;
diff --git a/sysdeps/unix/sysv/linux/tile/setcontext.S b/sysdeps/unix/sysv/linux/tile/setcontext.S
index ee9edbefe1bb..02d6a1578c75 100644
--- a/sysdeps/unix/sysv/linux/tile/setcontext.S
+++ b/sysdeps/unix/sysv/linux/tile/setcontext.S
@@ -190,7 +190,9 @@ END (__setcontext)
  weak_alias (__setcontext, setcontext)
ENTRY (__startcontext)
+	cfi_undefined (lr)
+	jalr r31
  	BEQZ r30, 1f
  	 move r0, r30

Chris Metcalf, EZChip Semiconductor

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