On Fri, May 28, 2021 at 02:22:01PM +0000, Justin He wrote: > > On Fri, May 28, 2021 at 07:39:50PM +0800, Jia He wrote: > > > We have '%pD' for printing a filename. It may not be perfect (by > > > default it only prints one component.) > > > > > > As suggested by Linus at [1]: > > > A dentry has a parent, but at the same time, a dentry really does > > > inherently have "one name" (and given just the dentry pointers, you > > > can't show mount-related parenthood, so in many ways the "show just > > > one name" makes sense for "%pd" in ways it doesn't necessarily for > > > "%pD"). But while a dentry arguably has that "one primary component", > > > a _file_ is certainly not exclusively about that last component. > > > > > > Hence "file_dentry_name()" simply shouldn't use "dentry_name()" at all. > > > Despite that shared code origin, and despite that similar letter > > > choice (lower-vs-upper case), a dentry and a file really are very > > > different from a name standpoint. > > > > > > Here stack space is preferred for file_d_path_name() because it is > > > much safer. The stack size 256 is a compromise between stack overflow > > > and too short full path. > > > > How is it "safer"? You already have a buffer passed from the caller. > > Are you saying that d_path_fast() might overrun a really small buffer > > but won't overrun a 256 byte buffer? > No, it won't overrun a 256 byte buf. When the full path size is larger than 256, the p->len is < 0 in prepend_name, and this overrun will be > dectected in extract_string() with "-ENAMETOOLONG". > > Each printk contains 2 vsnprintf. vsnprintf() returns the required size after formatting the string. > 1. vprintk_store() will invoke 1st vsnprintf() will 8 bytes space to get the reserve_size. In this case, the _buf_ could be less than _end_ by design. > 2. Then it invokes 2nd printk_sprint()->vscnprintf()->vsnprintf() to really fill the space. I think you need to explain _that_ in the commit log, not make some nebulous claim of "safer". > If we choose the stack space, it can meet above 2 cases. > > If we pass the parameter like: > p = d_path_fast(path, buf, end - buf); > We need to handle the complicated logic in prepend_name() > I have tried this way in local test, the code logic is very complicated > and not so graceful. > e.g. I need to firstly go through the loop and get the full path size of > that file. And then return reserved_size for that 1st vsnprintf I'm not sure why it's so complicated. p->len records how many bytes are needed for the entire path; can't you just return -p->len ?
