Users reported audio breakage for 32-bit pulse clients connected
to a 64-bit server over memfds. Investigating the issue further,
the problem is twofold:
1. iochannel's file-descriptor passing code is liberal in what it
issues: produced ancillary data object's "data" section exceeds
length field. How such an extra space is handled is a grey area
in the POSIX.1g spec, the IETF RFC #2292 "Advanced Sockets API
for IPv6" memo, and the cmsg(3) manpage.
2. A 64-bit kernel handling of such extra space differs by whether
the app is 64-bit or 32-bit. For 64-bit apps, the kernel
smartly ducks the issue. For 32-bit apps, an -EINVAL is
directly returned; that's due to a kernel CMSG header traversal
bug in the networking stack "32-bit sockets emulation layer".
Compare Linux Kernel's socket.h cmsg_nxthdr() code and the
32-bit emulation layer version of it at net/compat.c
cmsg_compat_nxthdr() for further info. Notice how the former
graciously ignores incomplete CMSGs while the latter _directly_
complains about them -- as of kernel version 4.9-rc5.
(A kernel patch is to be submitted)
Details:
iochannel typically uses sendmsg() for passing FDs & credentials.
>From RFC 2292, sendmsg() control data is just a heterogeneous
array of embedded ancillary objects that can differ in length.
Linguistically, a "control message" is an ancillary data object.
For example, below is a sendmsg() "msg_control" containing two
ancillary objects:
|<---------------------- msg_controllen---------------------->|
| |
|<--- ancillary data object -->|<----- ancillary data object->|
|<------- CMSG_SPACE() ------->|<------- CMSG_SPACE() ------->|
| | |
|<-------- cmsg_len ------->| |<-------- cmsg_len ------->| |
|<------- CMSG_LEN() ------>| |<------- CMSG_LEN() ------>| |
| | | | |
+-----+-----+-----+--+------+--+-----+-----+-----+--+------+--+
|cmsg_|cmsg_|cmsg_|XX|cmsg_ |XX|cmsg_|cmsg_|cmsg_|XX|cmsg_ |XX|
|len |level|type |XX|data[]|XX|len |level|type |XX|data[]|XX|
+-----+-----+-----+--+------+--+-----+-----+-----+--+----+-+--+
^^^^^^^ Ancil Object #1 ^^^^^^^ Ancil Object #2
(control message) (control message)
^
|
+--- sendmsg() "msg_control" points here
Problem is, while passing FDs, iochannel's code try to avoid
variable-length arrays by creating a single cmsg object that can
fit as much FDs as possible:
union {
struct cmsghdr hdr;
uint8_t data[CMSG_SPACE(sizeof(int) * MAX_ANCIL_DATA_FDS)];
} cmsg; ^^^^^^^^^^^^^^^^^^
Most of the time though the number of FDs to be passed is less
than the maximum above, thus "cmsg_len" is set to the _actual_ FD
array size:
cmsg.hdr.cmsg_len = CMSG_LEN(sizeof(int) * nfd);
^^^
This inconsistency tricks the kernel into thinking that we have 2
ancillay data objects instead of one! First cmsg is valid as
intended, but the second is instantly _corrupt_ since it has a
cmsg_len size of 0 -- thus failing kernel's CMSG_OK() tests.
For 32-bit apps on a 32-bit kernel, and 64-bit apps over a 64-bit
one, the kernel's own CMSG header traversal macros just ignore the
second "incomplete" cmsg. For 32-bit apps over a 64-bit kernel
though, the kernel 32-bit socket emulation macros does not forgive
such incompleteness and directly complains of invalid args (due to
a subtle bug).
Avoid this ugly problem, which can also bite us in a pure 64-bit
environment if MAX_ANCIL_DATA_FDS got extended to 5 FDs, by
setting "cmsg_data[]" array size to "cmsg_len".
BugLink: https://bugs.freedesktop.org/show_bug.cgi?id=97769
Signed-off-by: Ahmed S. Darwish <darwish.07 at gmail.com>
---
Hopefully variable-length arrays won't be problematic in esoteric
build environments?
src/pulsecore/iochannel.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/src/pulsecore/iochannel.c b/src/pulsecore/iochannel.c
index e62750b..8ace297 100644
--- a/src/pulsecore/iochannel.c
+++ b/src/pulsecore/iochannel.c
@@ -346,6 +346,8 @@ ssize_t pa_iochannel_write_with_creds(pa_iochannel*io, const void*data, size_t l
return r;
}
+/* For more details on FD passing, check the cmsg(3) manpage
+ * and IETF RFC #2292: "Advanced Sockets API for IPv6" */
ssize_t pa_iochannel_write_with_fds(pa_iochannel*io, const void*data, size_t l, int nfd, const int *fds) {
ssize_t r;
int *msgdata;
@@ -353,7 +355,7 @@ ssize_t pa_iochannel_write_with_fds(pa_iochannel*io, const void*data, size_t l,
struct iovec iov;
union {
struct cmsghdr hdr;
- uint8_t data[CMSG_SPACE(sizeof(int) * MAX_ANCIL_DATA_FDS)];
+ uint8_t data[CMSG_SPACE(sizeof(int) * nfd)];
} cmsg;
pa_assert(io);
--
2.4.3
