Clarify what abort_on_misalignment() does.

It doesn't request byte misalignment repair, it requests that byte
misalignment kill the program with SIGBUS; on platforms that don't
support aligned loads, we should be fetching possibly-misaligned data
using some safe instruction sequence, not by doing misaligned loads and
relying on them to trap to the kernel and be (slowly) emulated.

diff --git a/machdep.c b/machdep.c
index 3b8c9664f26669a38e5aaea3fc6903e61ece4e1c..7b259ae1c432089d714692ea3fe8cc7626e81b69 100644 (file)
--- a/machdep.c
+++ b/machdep.c
@@ -50,6 +50,15 @@ int snprintf(char *, size_t, const char *, ...)
 
 #include "machdep.h"
 
+/*
+ * On platforms where the CPU doesn't support unaligned loads, force
+ * unaligned accesses to abort with SIGBUS, rather than being fixed
+ * up (slowly) by the OS kernel; on those platforms, misaligned accesses
+ * are bugs, and we want tcpdump to crash so that the bugs are reported.
+ *
+ * The only OS on which this is necessary is DEC OSF/1^W^WDigital
+ * UNIX^W^WTru64 UNIX.
+ */
 int
 abort_on_misalignment(char *ebuf _U_, size_t ebufsiz _U_)
 {

diff --git a/tcpdump.c b/tcpdump.c
index 8d615d74cb8c2bbc3097521b855e5143193845bb..de0e320e675a7486a7e85a4eb527a77eacbdc2ee 100644 (file)
--- a/tcpdump.c
+++ b/tcpdump.c
@@ -951,6 +951,13 @@ main(int argc, char **argv)
        else
                program_name = argv[0];
 
+       /*
+        * On platforms where the CPU doesn't support unaligned loads,
+        * force unaligned accesses to abort with SIGBUS, rather than
+        * being fixed up (slowly) by the OS kernel; on those platforms,
+        * misaligned accesses are bugs, and we want tcpdump to crash so
+        * that the bugs are reported.
+        */
        if (abort_on_misalignment(ebuf, sizeof(ebuf)) < 0)
                error("%s", ebuf);