[newlib-cygwin] Cygwin: cygtls: fix context alignment

Corinna Vinschen corinna@sourceware.org
Mon Aug 29 13:21:23 GMT 2022


commit dcab768cb93e59712af5818736c4de783ae2c612
Author: Corinna Vinschen <corinna@vinschen.de>
Date:   Mon Aug 29 15:18:53 2022 +0200

    Cygwin: cygtls: fix context alignment
    A hang was encountered, apparently triggered by commit 63b503916d42,
    changing tls_pathbufs from malloc'ed to HeapAlloc'ed memory.  After
    lengthy debugging it transpired that adding the heap handle to the
    tls_pathbuf struct added 8 bytes to the cygtls area, thus moving
    the "context" member by 8 bytes, too, so it was suddently unaligned.
    Fix this for now by changing the alignment.
    Fix this once and for all, by adding code to the gentls_offsets script
    to check if the alignment of the "context" member is 16 bytes.  If not,
    print a matching error message, remove the just generated file, and exit
    with error.
    FIXME: It would be really nice to find a way to auomate the correct
    alignment of the "context" member, but I don't see any way to use
    alignment attributes to get what we need here.
    Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

 winsup/cygwin/local_includes/cygtls.h | 7 +++++--
 winsup/cygwin/scripts/gentls_offsets  | 9 +++++++++
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/winsup/cygwin/local_includes/cygtls.h b/winsup/cygwin/local_includes/cygtls.h
index 018a59946..61c4cbeef 100644
--- a/winsup/cygwin/local_includes/cygtls.h
+++ b/winsup/cygwin/local_includes/cygtls.h
@@ -178,11 +178,14 @@ public: /* Do NOT remove this public: line, it's a marker for gentls_offsets. */
   siginfo_t *sigwait_info;
   HANDLE signal_arrived;
   bool will_wait_for_signal;
+#if 0
   long __align;			/* Needed to align context to 16 byte. */
   /* context MUST be aligned to 16 byte, otherwise RtlCaptureContext fails.
      If you prepend cygtls members here, make sure context stays 16 byte
-     aligned. */
-  ucontext_t context;
+     aligned. The gentls_offsets script checks for that now and fails
+     if the alignment is wrong. */
+  ucontext_t __attribute__((__aligned__(__alignof__(long double)))) context;
   DWORD thread_id;
   siginfo_t infodata;
   struct pthread *tid;
diff --git a/winsup/cygwin/scripts/gentls_offsets b/winsup/cygwin/scripts/gentls_offsets
index 0adb702a3..040b5de6b 100755
--- a/winsup/cygwin/scripts/gentls_offsets
+++ b/winsup/cygwin/scripts/gentls_offsets
@@ -98,3 +98,12 @@ gawk -v start_offset="$start_offset" '\
 ' ${tmp_file} > "${output_file}"
+# Check if the `context' member is 16 bytes aligned.  Delete output_file
+# and bail out with error if not.
+MOD=$(awk '/_cygtls.context_p/{ print $3 % 16; }' "${output_file}")
+if [ $MOD -ne 0 ]
+  echo "Error: _cygtls.context member is not 16 bytes aligned!"
+  rm "${output_file}"
+  exit 1

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