sigaction.2, signal.7: Document kernel bugs in delivery of signals from CPU exceptions

signal.7: Which signal is delivered in response to a CPU exception
is under-documented and does not always make sense.  See
<https://bugzilla.kernel.org/show_bug.cgi?id=205831> for an
example where it doesn’t make sense; per the discussion there,
this cannot be changed because of backward compatibility concerns,
so let’s instead document the problem.

sigaction.2: For related reasons, the kernel doesn’t always fill
in all of the fields of the siginfo_t when delivering signals from
CPU exceptions.  Document this as well.  I imagine this one
_could_ be fixed, but the problem would still be relevant to
anyone using an older kernel.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

diff --git a/man2/sigaction.2 b/man2/sigaction.2
index bb7b7c9286e85c789fedb1a9665d55c5d2f95f61..8a04703b0eb5b1aca0403045f2146e43713ed932 100644 (file)
--- a/man2/sigaction.2
+++ b/man2/sigaction.2
@@ -1019,6 +1019,14 @@ handler.
 See the relevant Linux kernel sources for details.
 This use is obsolete now.
 .SH BUGS
+When delivering a signal with a
+.B SA_SIGINFO
+handler,
+the kernel does not always provide meaningful values
+for all of the fields of the
+.I siginfo_t
+that are relevant for that signal.
+.PP
 In kernels up to and including 2.6.13, specifying
 .B SA_NODEFER
 in

diff --git a/man7/signal.7 b/man7/signal.7
index a0406d6493270dd30ad5c97d26584d54b81804ee..2bcec4ad5b380807f3f6ca7717515fe8dfabd491 100644 (file)
--- a/man7/signal.7
+++ b/man7/signal.7
@@ -796,16 +796,36 @@ Linux 2.4 and earlier:
 .BR nanosleep (2).
 .SH CONFORMING TO
 POSIX.1, except as noted.
-.\" It must be a *very* long time since this was true:
-.\" .SH BUGS
-.\" .B SIGIO
-.\" and
-.\" .B SIGLOST
-.\" have the same value.
-.\" The latter is commented out in the kernel source, but
-.\" the build process of some software still thinks that
-.\" signal 29 is
-.\" .BR SIGLOST .
+.SH BUGS
+There are six signals that can be delivered
+as a consequence of a hardware exception:
+.BR SIGBUS ,
+.BR SIGEMT ,
+.BR SIGFPE ,
+.BR SIGILL ,
+.BR SIGSEGV ,
+and
+.BR SIGTRAP .
+Which of these signals is delivered,
+for any given hardware exception,
+is not documented and does not always make sense.
+.PP
+For example, an invalid memory access that causes delivery of
+.B SIGSEGV
+on one CPU architecture may cause delivery of
+.B SIGBUS
+on another architecture, or vice versa.
+.PP
+For another example, using the x86
+.I int
+instruction with a forbidden argument
+(any number other than 3 or 128)
+causes delivery of
+.BR SIGSEGV ,
+even though
+.B SIGILL
+would make more sense,
+because of how the CPU reports the forbidden operation to the kernel.
 .SH NOTES
 For a discussion of async-signal-safe functions, see
 .BR signal-safety (7).