Since commit cd678fce4280 ("switch logger to ->write_iter()"), any attempt to write to the log results in the log data being written over its own metadata, thus rendering the log unreadable.

The problem was first detected when I ran an Android userspace on the v3.18-rc1 kernel. However the issue can also be observed with a non-Android userspace by using echo/cat to write to/from /dev/log_main .

This patch resolves the problem by using a temporary to track the status of not-yet-committed writes to the log buffer.

Signed-off-by: Daniel Thompson daniel.thompson@linaro.org Cc: Al Viro viro@zeniv.linux.org.uk --- drivers/staging/android/logger.c | 13 ++++++++----- 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/drivers/staging/android/logger.c b/drivers/staging/android/logger.c index 28b93d3..a673ffa 100644 --- a/drivers/staging/android/logger.c +++ b/drivers/staging/android/logger.c @@ -420,7 +420,7 @@ static ssize_t logger_write_iter(struct kiocb *iocb, struct iov_iter *from) struct logger_log *log = file_get_log(iocb->ki_filp); struct logger_entry header; struct timespec now; - size_t len, count; + size_t len, count, w_off;

count = min_t(size_t, iocb->ki_nbytes, LOGGER_ENTRY_MAX_PAYLOAD);

@@ -452,11 +452,14 @@ static ssize_t logger_write_iter(struct kiocb *iocb, struct iov_iter *from) memcpy(log->buffer + log->w_off, &header, len); memcpy(log->buffer, (char *)&header + len, sizeof(header) - len);

- len = min(count, log->size - log->w_off); + /* Work with a copy until we are ready to commit the whole entry */ + w_off = logger_offset(log, log->w_off + sizeof(struct logger_entry));

- if (copy_from_iter(log->buffer + log->w_off, len, from) != len) { + len = min(count, log->size - w_off); + + if (copy_from_iter(log->buffer + w_off, len, from) != len) { /* - * Note that by not updating w_off, this abandons the + * Note that by not updating log->w_off, this abandons the * portion of the new entry that *was* successfully * copied, just above. This is intentional to avoid * message corruption from missing fragments. @@ -470,7 +473,7 @@ static ssize_t logger_write_iter(struct kiocb *iocb, struct iov_iter *from) return -EFAULT; }

- log->w_off = logger_offset(log, log->w_off + count); + log->w_off = logger_offset(log, w_off + count); mutex_unlock(&log->mutex);

/* wake up any blocked readers */ -- 1.9.3