On Thu, Jul 11, 2024 at 09:17:54PM -0400, Steven Rostedt wrote: > That "f->f_path.dentry" is a dereference of the passed in pointer. If we > did that in the TP_printk(), then it would dereference that file pointer > saved by the trace. This would happen at some time later from when the file > pointer was saved. That is, it will dereference the pointer when the user > reads the trace, not when the trace occurred. This could be seconds, > minutes, hours, days even months later! So %pD would not work there. Indeed. I'm so used to these useful format strings that I keep forgetting about them doing non-trivial things. Which also brings up that it would be good if we had some kind of static checker that detects usage of these magic %p extensions in the trace macros and warns about them. > __dynamic_array(char, pathname, snprintf(NULL, 0, "%pD", xf->file) + 1); > > // This will allocated the space needed for the string > > sprintf(__get_dynamic_array(pathname), "%pD", xf->file); > > // and the above will copy it directly to that location. > // It assumes the value of the first snprintf() will be the same as the second. > > (char *)__get_dynamic_array(pathname), > > // for accessing the string, although yes, __get_str(pathname) would work, > // but that's more by luck than design. That sounds pretty cool, but except for the dynamic sizing doesn't really buy us much over the version Darrick proposed, right? > Looking at this file, I noticed that you have some open coded __string_len() > fields. Why not just use that? In fact, I think I even found a bug: > > There's a: > memcpy(__get_str(name), name, name->len); > > Where I think it should have been: > > memcpy(__get_str(name), name->name, name->len); > > Hmm, I should make sure that __string() and __string_len() are passed in > strings. As this is a common bug. > > I can make this a formal patch if you like. Although, I haven't even tried > compile testing it ;-) Without having compiled it either, this looks sensible to me. As XFS was one of the earliest adopters of event tracing I suspect these predate the string helpers.
