3.16.51-rc1 review patch. If anyone has any objections, please let me know.
------------------
From: "Eric W. Biederman" ebiederm@xmission.com
commit d08477aa975e97f1dc64c0ae59cebf98520456ce upstream.
We have a weird and problematic intersection of features that when they all come together result in ambiguous siginfo values, that we can not support properly.
- Supporting fcntl(F_SETSIG,...) with arbitrary valid signals.
- Using positive values for POLL_IN, POLL_OUT, POLL_MSG, ..., etc that imply they are signal specific si_codes and using the aforementioned arbitrary signal to deliver them.
- Supporting injection of arbitrary siginfo values for debugging and checkpoint/restore.
The result is that just looking at siginfo si_codes of 1 to 6 are ambigious. It could either be a signal specific si_code or it could be a generic si_code.
For most of the kernel this is a non-issue but for sending signals with siginfo it is impossible to play back the kernel signals and get the same result.
Strictly speaking when the si_code was changed from SI_SIGIO to POLL_IN and friends between 2.2 and 2.4 this functionality was not ambiguous, as only real time signals were supported. Before 2.4 was released the kernel began supporting siginfo with non realtime signals so they could give details of why the signal was sent.
The result is that if F_SETSIG is set to one of the signals with signal specific si_codes then user space can not know why the signal was sent.
I grepped through a bunch of userspace programs using debian code search to get a feel for how often people choose a signal that results in an ambiguous si_code. I only found one program doing so and it was using SIGCHLD to test the F_SETSIG functionality, and did not appear to be a real world usage.
Therefore the ambiguity does not appears to be a real world problem in practice. Remove the ambiguity while introducing the smallest chance of breakage by changing the si_code to SI_SIGIO when signals with signal specific si_codes are targeted.
Fixes: v2.3.40 -- Added support for queueing non-rt signals Fixes: v2.3.21 -- Changed the si_code from SI_SIGIO Signed-off-by: "Eric W. Biederman" ebiederm@xmission.com Signed-off-by: Ben Hutchings ben@decadent.org.uk --- fs/fcntl.c | 13 ++++++++++++- include/linux/signal.h | 8 ++++++++ include/uapi/asm-generic/siginfo.h | 4 ++-- 3 files changed, 22 insertions(+), 3 deletions(-)
--- a/fs/fcntl.c +++ b/fs/fcntl.c @@ -472,10 +472,21 @@ static void send_sigio_to_task(struct ta si.si_signo = signum; si.si_errno = 0; si.si_code = reason; + /* + * Posix definies POLL_IN and friends to be signal + * specific si_codes for SIG_POLL. Linux extended + * these si_codes to other signals in a way that is + * ambiguous if other signals also have signal + * specific si_codes. In that case use SI_SIGIO instead + * to remove the ambiguity. + */ + if (sig_specific_sicodes(signum)) + si.si_code = SI_SIGIO; + /* Make sure we are called with one of the POLL_* reasons, otherwise we could leak kernel stack into userspace. */ - BUG_ON((reason & __SI_MASK) != __SI_POLL); + BUG_ON((reason < POLL_IN) || ((reason - POLL_IN) >= NSIGPOLL)); if (reason - POLL_IN >= NSIGPOLL) si.si_band = ~0L; else --- a/include/linux/signal.h +++ b/include/linux/signal.h @@ -424,10 +424,18 @@ int unhandled_signal(struct task_struct rt_sigmask(SIGCONT) | rt_sigmask(SIGCHLD) | \ rt_sigmask(SIGWINCH) | rt_sigmask(SIGURG) )
+#define SIG_SPECIFIC_SICODES_MASK (\ + rt_sigmask(SIGILL) | rt_sigmask(SIGFPE) | \ + rt_sigmask(SIGSEGV) | rt_sigmask(SIGBUS) | \ + rt_sigmask(SIGTRAP) | rt_sigmask(SIGCHLD) | \ + rt_sigmask(SIGPOLL) | rt_sigmask(SIGSYS) | \ + SIGEMT_MASK ) + #define sig_kernel_only(sig) siginmask(sig, SIG_KERNEL_ONLY_MASK) #define sig_kernel_coredump(sig) siginmask(sig, SIG_KERNEL_COREDUMP_MASK) #define sig_kernel_ignore(sig) siginmask(sig, SIG_KERNEL_IGNORE_MASK) #define sig_kernel_stop(sig) siginmask(sig, SIG_KERNEL_STOP_MASK) +#define sig_specific_sicodes(sig) siginmask(sig, SIG_SPECIFIC_SICODES_MASK)
#define sig_user_defined(t, signr) \ (((t)->sighand->action[(signr)-1].sa.sa_handler != SIG_DFL) && \ --- a/include/uapi/asm-generic/siginfo.h +++ b/include/uapi/asm-generic/siginfo.h @@ -161,7 +161,7 @@ typedef struct siginfo { #define SI_TIMER __SI_CODE(__SI_TIMER,-2) /* sent by timer expiration */ #define SI_MESGQ __SI_CODE(__SI_MESGQ,-3) /* sent by real time mesq state change */ #define SI_ASYNCIO -4 /* sent by AIO completion */ -#define SI_SIGIO -5 /* sent by queued SIGIO */ +#define SI_SIGIO __SI_CODE(__SI_POLL,-5) /* sent by queued SIGIO */ #define SI_TKILL -6 /* sent by tkill system call */ #define SI_DETHREAD -7 /* sent by execve() killing subsidiary threads */
@@ -234,7 +234,7 @@ typedef struct siginfo { #define NSIGCHLD 6
/* - * SIGPOLL si_codes + * SIGPOLL (or any other signal without signal specific si_codes) si_codes */ #define POLL_IN (__SI_POLL|1) /* data input available */ #define POLL_OUT (__SI_POLL|2) /* output buffers available */