From: Tom Hughes <tom@compton.nu>
Date: Sat, 5 Nov 2005 00:17:21 +0000 (+0000)
Subject: Add a comment documenting the reason for subtracting one from the IP value
X-Git-Tag: svn/VALGRIND_3_1_0~220
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=efaaa0b74a695a68ef5e758102d3fe3920e121fd;p=thirdparty%2Fvalgrind.git

Add a comment documenting the reason for subtracting one from the IP value
after each pass of the loop.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@5012
---

diff --git a/coregrind/m_stacktrace.c b/coregrind/m_stacktrace.c
index 62b83c29da..c14b57e347 100644
--- a/coregrind/m_stacktrace.c
+++ b/coregrind/m_stacktrace.c
@@ -110,6 +110,21 @@ UInt VG_(get_StackTrace2) ( Addr* ips, UInt n_ips,
    ips[0] = ip;
    i = 1;
 
+   /* Loop unwinding the stack. Note that the IP value we get on
+    * each pass (whether from CFI info or a stack frame) is a
+    * return address so is actually after the calling instruction
+    * in the calling function.
+    *
+    * Because of this we subtract one from the IP after each pass
+    * of the loop so that we find the right CFI block on the next
+    * pass - otherwise we can find the wrong CFI info if it happens
+    * to change after the calling instruction and that will mean
+    * that we will fail to unwind the next step.
+    *
+    * This most frequently happens at the end of a function when
+    * a tail call occurs and we wind up using the CFI info for the
+    * next function which is completely wrong.
+    */
    while (True) {
 
       if (i >= n_ips)