> On Jun 28, 2018, at 12:36 AM, Warner Losh <i...@bsdimp.com> wrote: > > > > On Thu, Jun 28, 2018 at 12:54 AM, Devin Teske <dte...@freebsd.org > <mailto:dte...@freebsd.org>> wrote: > >> On Jun 27, 2018, at 7:35 PM, Warner Losh <i...@bsdimp.com >> <mailto:i...@bsdimp.com>> wrote: >> >> >> >> On Wed, Jun 27, 2018 at 8:14 PM, Devin Teske <dte...@freebsd.org >> <mailto:dte...@freebsd.org>> wrote: >> >>> On Jun 27, 2018, at 6:58 PM, Warner Losh <i...@bsdimp.com >>> <mailto:i...@bsdimp.com>> wrote: >>> >>> >>> >>> On Wed, Jun 27, 2018 at 7:49 PM, Devin Teske <dte...@freebsd.org >>> <mailto:dte...@freebsd.org>> wrote: >>> >>>> On Jun 27, 2018, at 5:59 PM, Warner Losh <i...@bsdimp.com >>>> <mailto:i...@bsdimp.com>> wrote: >>>> >>>> >>>> >>>> On Wed, Jun 27, 2018 at 5:52 PM, Gleb Smirnoff <gleb...@freebsd.org >>>> <mailto:gleb...@freebsd.org>> wrote: >>>> On Wed, Jun 27, 2018 at 04:11:09AM +0000, Warner Losh wrote: >>>> W> Author: imp >>>> W> Date: Wed Jun 27 04:11:09 2018 >>>> W> New Revision: 335690 >>>> W> URL: https://svnweb.freebsd.org/changeset/base/335690 >>>> <https://svnweb.freebsd.org/changeset/base/335690> >>>> W> >>>> W> Log: >>>> W> Fix devctl generation for core files. >>>> W> >>>> W> We have a problem with vn_fullpath_global when the file exists. Work >>>> W> around it by printing the full path if the core file name starts with >>>> /, >>>> W> or current working directory followed by the filename if not. >>>> >>>> Is this going to work when a core is dumped not at current working >>>> directory, >>>> but at absolute path? e.g. kern.corefile=/var/log/cores/%N.core >>>> >>>> Yes. That works. >>>> >>>> Looks like the vn_fullpath_global needs to be fixed rather than problem >>>> workarounded. >>>> >>>> It can't be fixed reliably. FreeBSD does not and cannot map a vnode to a >>>> name. The only reason we're able to at all is due to the name cache. And >>>> when we recreate a file, we invalidate the name cache. And even if we >>>> fixed that, there's no guarantee the name cache won't get flushed before >>>> we translate the name.... Linux can do this because it keeps the path name >>>> associated with the inode. FreeBSD simply doesn't. >>>> >>> >>> They said it couldn't be done, but I personally have done it ... >>> >>> I map vnodes to full paths in dwatch. It's not impossible, just implausibly >>> hard. >>> >>> I derived my formula by reading the C code which was very twisty-turny and >>> rather hard to read at times (and I have been using C since 1998 on >>> everything from AIX and OSF/1 to HP/UX, Cygwin, MinGW, FreeBSD, countless >>> Linux-like things, IRIX, and a God-awful remainder of many others; the >>> vnode code was an adventure to say the least). >>> >>> You're welcome to see how it's done in /usr/libexec/dwatch/vop_create >>> >>> I load up a clause-local variable named "path" with a path constructed from >>> a pointer to a vnode structure argument to VOP_CREATE(9). >>> >>> The D code could easily be rewritten back into C to walk the vnode to >>> completion (compared to the D code which is bounded by depth-limit since >>> DTrace doesn't provide loops; so you have to, as I have done, use a >>> higher-level language wrapper to repeat the code to some desired limit; >>> dwatch defaulting to 64 for directory depth limit). >>> >>> Compared this, say, to vfssnoop.d from Chapter 5 of the DTrace book which >>> utilizes the vfs:namei:lookup: probes. My approach in dwatch is far more >>> accurate, produces full paths, and I've benchmarked it at lower overhead >>> with better results. >>> >>> Maybe some of this can be useful? If not, just ignore me. >>> >>> IMHO, there's no benefit from the crazy hoops than the super simple >>> heuristic I did: if the path starts with / print it. If the path doesn't >>> print cwd / and then the path. >>> >>> I look forward to seeing your conversion of the D to C that works, even >>> when there's namespace cache pressure. >>> >> >> I was looking at the output of "dwatch -dX vop_create" (the -d flag to >> dwatch causes the generated dwatch to be printed instead of executed) and >> thinking to myself: >> >> I could easily convert this, as I can recognize the flattened loop >> structure. However, not many people will be able to do it. I wasn't really >> volunteering, but now I'm curious what other people see when they run >> "dwatch -dX vop_create". If it's half as bad as I think it is, then I'll >> gladly do it -- but will have to balance it against work priorities. >> >> We don't have the data you do in vop_create anymore, just the vp at the >> point in the code where we want to do the reverse translation... But we also >> have a name that's been filled in from the template and cwd, which gives us >> almost the same thing as we'd hoped to dig out of the name cache... >> > > Given: > > struct vnode *vp, const char *fi_name > > Here's a rough (not optimized; unchecked) crack at a C equivalent: > > Thanks, but this code suffers from the same problem that vn_fullpath_global() > suffers from: we clear out parts of the cache when we open an existing file > with O_CREAT. This one NULL leads to a cascade of failures, as show below. > > > > #include <sys/mount.h> > #include <sys/vnode.h> > > #include <limits.h> > > int i, max_depth = 64; > char *d_name = NULL; > char *fi_fs = NULL; > char *fi_mount = NULL; > char *cp; > struct namecache *ncp = NULL; > struct mount *mount = NULL; > struct vnode *dvp = NULL; > char *nc[max_depth+1]; > char path[PATH_MAX] = __DECONST(char *, ""); > > if (vp != NULL) { > ncp = vp->v_cache_dst.tqh_first; > > For my case, this is NULL since we clear out the cache when we re-create the > file. I confirmed this by adding a printf where I put in my workaround and I > get: > vp->v_cache_dst.tqh_first is 0xfffff80003bd7a10 (first time, no cat.core > exists) > vp->v_cache_dst.tqh_first is 0 (second time, cat.core does exist) > > mount = vp->v_mount; /* ptr to vfs we are in */ > if (vp->v_cache_dd != NULL) > d_name = vp->v_cache_dd->nc_name; > } > if (mount != NULL) { > fi_fs = mount->mnt_stat.f_fstypename; > if (fi_fs != NULL && *fi_fs != '\0') { > if (!strcmp(fi_fs, "devfs")) ncp = NULL; > } else { > if (fi_name == NULL || *fi_name == '\0') ncp = NULL; > } > fi_mount = mount->mnt_stat.f_mntonname; > } else { > ncp = NULL; > } > for (i = 0; i <= max_depth; i++) { > nc[i] = NULL; > } > if (ncp != NULL) { > /* Reach into vnode of parent of name */ > if (ncp->nc_dvp != NULL) > dvp = ncp->nc_dvp->v_cache_dst.tqh_first; > if (dvp != NULL) nc[0] = dvp->nc_name; > } > > since ncp is NULL here, we never set nc[0] > > > if (nc[0] == NULL || *nc[0] == '\0' || !strcmp(nc[0], "/")) > dvp = NULL; > > so dvp is NULL > > /* > * BEGIN Pathname-depth loop > */ > for (i = 1; i < max_depth; i++) { > if (dvp == NULL) break; > > and we break out of this loop without setting any other nc[] entries. > > if (dvp->nc_dvp != NULL) > dvp = dvp->nc_dvp->v_cache_dst.tqh_first; > else > dvp = NULL; > if (dvp != NULL) nc[i] = dvp->nc_name; > } > /* > * END Pathname-depth loop > */ > if (dvp != NULL) { > if (dvp->nc_dvp != NULL) > dvp = dvp->nc_dvp->v_cache_dst.tqh_first; > else > dvp = NULL; > if (dvp != NULL && dvp->nc_dvp != NULL) > nc[max_depth] = __DECONST(char *, "..."); > } > if (fi_mount != NULL) { > /* > * Join full path > * NB: Up-to but not including the parent directory (joined > below) > */ > strcat(path, fi_mount); > if (strcmp(fi_mount, "/")) strcat(path, "/"); > for (i = max_depth; i >= 0; i--) { > char *namei = nc[i]; > strcat(path, namei == NULL ? "" : namei); > > which joins a lot of empty strings together > > if (namei != NULL) strcat(path, "/"); > } > /* Join the parent directory name */ > if (d_name != NULL && *d_name != '\0') { > strcat(path, d_name); > strcat(path, "/"); > } > /* Join the entry name */ > if (fi_name != NULL && *fi_name != '\0') > strcat(path, fi_name); > > since we don't have the fi_name from the VOP_CREATE where we want to do this, > we have nothing for here. > > } > > So I appreciate the effort here, but I'm not sure that it's useful in this > context. vn_fullpath* already can do this sort of thing, but can't in the > case where vp->v_cache_dst.tqh_first is NULL. >
Thank you so much for the thoughtful response. It means a lot really. -- Cheers, Devin _______________________________________________ svn-src-all@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/svn-src-all To unsubscribe, send any mail to "svn-src-all-unsubscr...@freebsd.org"
