Date: Wed, 27 Jun 2018 18:49:42 -0700 From: Devin Teske <dteske@FreeBSD.org> To: Warner Losh <imp@bsdimp.com> Cc: Devin Teske <dteske@FreeBSD.org>, Gleb Smirnoff <glebius@freebsd.org>, Warner Losh <imp@freebsd.org>, src-committers <src-committers@freebsd.org>, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r335690 - head/sys/kern Message-ID: <16B9F36D-E92A-4FEC-B096-5E24A4E180FC@FreeBSD.org> In-Reply-To: <CANCZdfpdBOin=w0qEGXM%2BciTAbETezYTOPq1nuEV1pT5QATEHA@mail.gmail.com> References: <201806270411.w5R4B9ZB078994@repo.freebsd.org> <20180627235216.GO1165@FreeBSD.org> <CANCZdfpdBOin=w0qEGXM%2BciTAbETezYTOPq1nuEV1pT5QATEHA@mail.gmail.com>
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> On Jun 27, 2018, at 5:59 PM, Warner Losh <imp@bsdimp.com> wrote: >=20 >=20 >=20 > On Wed, Jun 27, 2018 at 5:52 PM, Gleb Smirnoff <glebius@freebsd.org = <mailto:glebius@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>=20 > W> Log: > W> Fix devctl generation for core files. > W> =20 > 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. >=20 > Is this going to work when a core is dumped not at current working = directory, > but at absolute path? e.g. kern.corefile=3D/var/log/cores/%N.core >=20 > Yes. That works. > =20 > Looks like the vn_fullpath_global needs to be fixed rather than = problem > workarounded. >=20 > 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. >=20 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. --=20 Devin=
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