* [lttng-dev] Fix: application SIGBUS when starting in parallel with sessiond
2013-11-06 12:18 ` [lttng-dev] Fix: application SIGBUS when starting in parallel with sessiond Mathieu Desnoyers
@ 2013-11-06 15:50 ` David Goulet
0 siblings, 0 replies; 2+ messages in thread
From: David Goulet @ 2013-11-06 15:50 UTC (permalink / raw)
On 06 Nov (12:18:47), Mathieu Desnoyers wrote:
> Should be applied to master, stable-2.3, stable-2.2.
Merged!
>
> Thanks,
>
> Mathieu
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com
> commit be2bcc7e9a02e5bd78dc2615886af32ddd87032b
> Author: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
> Date: Wed Nov 6 07:06:51 2013 -0500
>
> Fix: application SIGBUS when starting in parallel with sessiond
>
> There is a race between application startup and sessiond startup, where
> there is an intermediate state where applications can SIGBUS if they see
> a zero-sized shm, if the shm has been created, but not ftruncated yet.
>
> On the sessiond side, we need to ensure that the shared memory is
> writeable by applications as long as its size is 0, which allow
> applications to perform ftruncate and extend its size.
>
> On the UST side, another commit needs to ensure that UST can read the
> shared memory file descriptor with a read() system call before they
> try accessing it through a memory map (which triggers the SIGBUS if
> the access goes beyond the file size)
>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
>
> diff --git a/src/bin/lttng-sessiond/shm.c b/src/bin/lttng-sessiond/shm.c
> index b94f4eb..7bb52d4 100644
> --- a/src/bin/lttng-sessiond/shm.c
> +++ b/src/bin/lttng-sessiond/shm.c
> @@ -48,40 +48,17 @@ static int get_wait_shm(char *shm_path, size_t mmap_size, int global)
> /* Default permissions */
> mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP;
>
> - /* Change owner of the shm path */
> + /*
> + * Change owner of the shm path.
> + */
> if (global) {
> - ret = chown(shm_path, 0, 0);
> - if (ret < 0) {
> - if (errno != ENOENT) {
> - PERROR("chown wait shm");
> - goto error;
> - }
> - }
> -
> /*
> - * If global session daemon, any application can register so the shm
> - * needs to be set in read-only mode for others.
> + * If global session daemon, any application can
> + * register. Make it initially writeable so applications
> + * registering concurrently can do ftruncate() by
> + * themselves.
> */
> - mode |= S_IROTH;
> - } else {
> - ret = chown(shm_path, getuid(), getgid());
> - if (ret < 0) {
> - if (errno != ENOENT) {
> - PERROR("chown wait shm");
> - goto error;
> - }
> - }
> - }
> -
> - /*
> - * Set permissions to the shm even if we did not create the shm.
> - */
> - ret = chmod(shm_path, mode);
> - if (ret < 0) {
> - if (errno != ENOENT) {
> - PERROR("chmod wait shm");
> - goto error;
> - }
> + mode |= S_IROTH | S_IWOTH;
> }
>
> /*
> @@ -107,13 +84,32 @@ static int get_wait_shm(char *shm_path, size_t mmap_size, int global)
> }
>
> #ifndef __FreeBSD__
> - ret = fchmod(wait_shm_fd, mode);
> - if (ret < 0) {
> - PERROR("fchmod");
> - exit(EXIT_FAILURE);
> + if (global) {
> + ret = fchown(wait_shm_fd, 0, 0);
> + if (ret < 0) {
> + PERROR("fchown");
> + exit(EXIT_FAILURE);
> + }
> + /*
> + * If global session daemon, any application can
> + * register so the shm needs to be set in read-only mode
> + * for others.
> + */
> + mode &= ~S_IWOTH;
> + ret = fchmod(wait_shm_fd, mode);
> + if (ret < 0) {
> + PERROR("fchmod");
> + exit(EXIT_FAILURE);
> + }
> + } else {
> + ret = fchown(wait_shm_fd, getuid(), getgid());
> + if (ret < 0) {
> + PERROR("fchown");
> + exit(EXIT_FAILURE);
> + }
> }
> #else
> -#warning "FreeBSD does not support setting file mode on shm FD. Remember that for secure use, lttng-sessiond should be started before applications linked on lttng-ust."
> +#warning "FreeBSD does not support setting file mode on shm FD."
> #endif
>
> DBG("Got the wait shm fd %d", wait_shm_fd);
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