Messages in this thread | | | Date | Tue, 3 Mar 2009 19:35:01 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH 5/5] tracing: add binary buffer files for use with splice |
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On Tue, 03 Mar 2009 21:49:26 -0500 Steven Rostedt <rostedt@goodmis.org> wrote:
> +static ssize_t > +tracing_buffers_read(struct file *filp, char __user *ubuf, > + size_t count, loff_t *ppos) > +{ > + struct ftrace_buffer_info *info = filp->private_data; > + unsigned int pos; > + ssize_t ret; > + size_t size; > + > + /* Do we have previous read data to read? */ > + if (info->read < PAGE_SIZE) > + goto read; > + > + info->read = 0; > + > + ret = ring_buffer_read_page(info->tr->buffer, > + &info->spare, > + count, > + info->cpu, 0); > + if (ret < 0) > + return 0; > + > + pos = ring_buffer_page_len(info->spare); > + > + if (pos < PAGE_SIZE) > + memset(info->spare + pos, 0, PAGE_SIZE - pos); > + > +read: > + size = PAGE_SIZE - info->read; > + if (size > count) > + size = count; > + > + ret = copy_to_user(ubuf, info->spare + info->read, size); > + if (ret) > + return -EFAULT;
Conventionally a read() system call will return the number of bytes copied, and will only return -EFOO if the number of bytes copied was zero.
Lots of parts of the kernel break this, but it's usually device drivers and scruffy pseudo files, in which case a partial file read doesn't make much sense. This doesn't make the broken behaviour right, but at least we have a bit of a weaselly excuse in that case.
> + *ppos += size; > + info->read += size; > + > + return size; > +}
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