| Message ID | 20230719221918.8937-1-krisman@suse.de (mailing list archive) |
|---|---|
| Headers | show |
| Series | Support negative dentries on case-insensitive ext4 and f2fs | expand |
Sorry, one more thing... On Wed, Jul 19, 2023 at 06:19:11PM -0400, Gabriel Krisman Bertazi wrote: > > Another problem exists when turning a negative dentry to positive. If > the negative dentry has a different case than what is currently being > used for lookup, the dentry cannot be reused without changing its name, > in order to guarantee filename-preserving semantics to userspace. We > need to either change the name or invalidate the dentry. This issue is > currently avoided in mainline, since the negative dentry mechanism is > disabled. Are you sure this problem even needs to be solved? It actually isn't specific to negative dentries. If you have a file "foo" that's not in the dcache, and you open it (or look it up in any other way) as "FOO", then the positive dentry that gets created is named "FOO". As a result, the name that shows up in /proc/$pid/fd/ for anyone who has the file open is "FOO", not the true name "foo". This is true even for processes that open it as "foo", as long as the dentry remains in the dcache. No negative dentries involved at all! Is your thinking that you just don't want to increase the number of ways in which this behavior can occur? Or, it looks like the positive dentry case is solvable using d_add_ci(). So maybe you are planning to do that? It's not clear to me. - Eric
Eric Biggers <ebiggers@kernel.org> writes: >> Another problem exists when turning a negative dentry to positive. If >> the negative dentry has a different case than what is currently being >> used for lookup, the dentry cannot be reused without changing its name, >> in order to guarantee filename-preserving semantics to userspace. We >> need to either change the name or invalidate the dentry. This issue is >> currently avoided in mainline, since the negative dentry mechanism is >> disabled. > > Are you sure this problem even needs to be solved? Yes, because we promise name-preserving semantics. If we don't do it, the name on the disk might be different than what was asked for, and tools that rely on it (they exist) will break. During initial testing, I've had tools breaking with case-insensitive ext4 because they created a file, did getdents and wanted to see exactly the name they used. > It actually isn't specific to negative dentries. If you have a file "foo" > that's not in the dcache, and you open it (or look it up in any other way) as > "FOO", then the positive dentry that gets created is named "FOO". > > As a result, the name that shows up in /proc/$pid/fd/ for anyone who has the > file open is "FOO", not the true name "foo". This is true even for processes > that open it as "foo", as long as the dentry remains in the dcache. > > No negative dentries involved at all! I totally agree it is goes beyond negative dentries, but this case is particularly important because it is the only one (that I know of) where the incorrect case might actually be written to the disk. other cases, like /proc/<pid>/fd/ can just display a different case to userspace, which is confusing. Still, the disk has the right version, exactly as originally created. I see the current /proc/$pid/fd/ semantics as a bug. In fact, I have/had a bug report for bwrap/flatkpak breaking because it was mounting something and immediately checking /proc/mounts to confirm it worked. A former team member tried fixing it a while ago, but we didn't follow up, and I don't really love the way they did it. I need to look into that. > Or, it looks like the positive dentry case is solvable using d_add_ci(). > So maybe you are planning to do that? It's not clear to me. I want to use d_add_ci for the future, yes. It is not hard, but not trivial, because there is an infinite recursion if d_add_ci uses ->d_compare() when doing the lookup for a duplicate. We sent a patch to fix d_add_ci a while ago, but it was rejected. I need to revisit.
On Thu, Jul 20, 2023 at 01:35:37PM -0400, Gabriel Krisman Bertazi wrote: > Eric Biggers <ebiggers@kernel.org> writes: > > >> Another problem exists when turning a negative dentry to positive. If > >> the negative dentry has a different case than what is currently being > >> used for lookup, the dentry cannot be reused without changing its name, > >> in order to guarantee filename-preserving semantics to userspace. We > >> need to either change the name or invalidate the dentry. This issue is > >> currently avoided in mainline, since the negative dentry mechanism is > >> disabled. > > > > Are you sure this problem even needs to be solved? > > Yes, because we promise name-preserving semantics. If we don't do it, > the name on the disk might be different than what was asked for, and tools > that rely on it (they exist) will break. During initial testing, I've > had tools breaking with case-insensitive ext4 because they created a > file, did getdents and wanted to see exactly the name they used. > > > It actually isn't specific to negative dentries. If you have a file "foo" > > that's not in the dcache, and you open it (or look it up in any other way) as > > "FOO", then the positive dentry that gets created is named "FOO". > > > > As a result, the name that shows up in /proc/$pid/fd/ for anyone who has the > > file open is "FOO", not the true name "foo". This is true even for processes > > that open it as "foo", as long as the dentry remains in the dcache. > > > > No negative dentries involved at all! > > I totally agree it is goes beyond negative dentries, but this case is > particularly important because it is the only one (that I know of) where > the incorrect case might actually be written to the disk. other cases, > like /proc/<pid>/fd/ can just display a different case to userspace, > which is confusing. Still, the disk has the right version, exactly as > originally created. > > I see the current /proc/$pid/fd/ semantics as a bug. In fact, I have/had > a bug report for bwrap/flatkpak breaking because it was mounting > something and immediately checking /proc/mounts to confirm it worked. A > former team member tried fixing it a while ago, but we didn't follow up, > and I don't really love the way they did it. I need to look into that. > > > Or, it looks like the positive dentry case is solvable using d_add_ci(). > > So maybe you are planning to do that? It's not clear to me. > > I want to use d_add_ci for the future, yes. It is not hard, but not > trivial, because there is an infinite recursion if d_add_ci uses > ->d_compare() when doing the lookup for a duplicate. We sent a patch to > fix d_add_ci a while ago, but it was rejected. I need to revisit. > Thanks, I missed that choosing a different-case dentry actually changes the name given to the new file. This is because the filesystem is told about the name of the file to create via the negative dentry that gets found/created -- not via the original user-specified string. It would help if you made this clear in a code comment. Taking the comment I suggested, I'd maybe revise it to: /* * If the lookup is for creation, then a negative dentry * can only be reused if it's a case-sensitive match, * not just a case-insensitive one. This is needed to * make the new file be created with the name the user * specified, preserving case. * - Eric
Hi, V3 applies the fixes suggested by Eric Biggers (thank you for your review!). Changelog inlined in the patches. Retested with xfstests for ext4 and f2fs. -- cover letter from v1. This patchset enables negative dentries for case-insensitive directories in ext4/f2fs. It solves the corner cases for this feature, including those already tested by fstests (generic/556). It also solves an existing bug with the existing implementation where old negative dentries are left behind after a directory conversion to case-insensitive. Testing-wise, I ran sanity checks to show it properly uses the created negative dentries, observed the expected performance increase of the dentry cache hit, and showed it survives the quick group in fstests on both f2fs and ext4 without regressions. * Background Negative dentries have always been disabled in case-insensitive directories because, in their current form, they can't provide enough assurances that all the case variations of a filename won't exist in a directory, and the name-preserving case-insenstive semantics during file creation prevents some negative dentries from being instantiated unmodified. Nevertheless, for the general case, the existing implementation would already work with negative dentries, even though they are fully disabled. That is: if the original lookup that created the dentry was done in a case-insensitive way, the negative dentry can usually be validated, since it assures that no other dcache entry exists, *and* that no variation of the file exists on disk (since the lookup failed). A following lookup would then be executed with the case-insensitive-aware d_hash and d_lookup, which would find the right negative dentry and use it. The first corner case arises when a case-insensitive directory has negative dentries that were created before the directory was flipped to case-insensitive. A directory must be empty to be converted, but it doesn't mean the directory doesn't have negative dentry children. If that happens, the dangling dentries left behind can't assure that no case-variation of the name exists. They only mean the exact name doesn't exist. A further lookup would incorrectly validate them. The code below demonstrates the problem. In this example $1 and $2 are two strings, where: (i) $1 != $2 (ii) casefold($1) == casefold($2) (iii) hash($1) == hash($2) == hash(casefold($1)) Then, the following sequence could potentially return a ENOENT, even though the case-insensitive lookup should exist: mkdir d <- Case-sensitive directory touch d/$1 touch d/$2 unlink d/$1 <- leaves negative dentry behind. unlink d/$2 <- leaves *another* negative dentry behind. chattr +F d <- make 'd' case-insensitive. touch d/$1 <- Both negative dentries could match. finds one of them, and instantiate access d/$1 <- Find the other negative dentry, get -ENOENT. In fact, this is a problem even on the current implementation, where negative dentries for CI are disabled. There was a bug reported by Al Viro in 2020, where a directory might end up with dangling negative dentries created during a case-sensitive lookup, because they existed before the +F attribute was set. It is hard to trigger the issue, because condition (iii) is hard to test on an unmodified kernel. By hacking the kernel to force the hash collision, there are a few ways we can trigger this bizarre behavior in case-insensitive directories through the insertion of negative dentries. Another problem exists when turning a negative dentry to positive. If the negative dentry has a different case than what is currently being used for lookup, the dentry cannot be reused without changing its name, in order to guarantee filename-preserving semantics to userspace. We need to either change the name or invalidate the dentry. This issue is currently avoided in mainline, since the negative dentry mechanism is disabled. * Proposal The main idea is to differentiate negative dentries created in a case-insensitive context from those created during a case-sensitive lookup via a new dentry flag, D_CASEFOLD_LOOKUP, set by the filesystem the d_lookup hook. Since the former can be used (except for the name-preserving issue), d_revalidate will just check the flag to quickly accept or reject the dentry. A different solution would be to guarantee no negative dentry exists during the case-sensitive to case-insensitive directory conversion (the other direction is safe). It has the following problems: 1) It is not trivial to implement a race-free mechanism to ensure negative dentries won't be recreated immediately after invalidation while converting the directory. 2) The knowledge whether the negative dentry is valid (i.e. comes from a case-insensitive lookup) is implicit on the fact that we are correctly invalidating dentries when converting the directory. Having a D_CASEFOLD_LOOKUP avoids both issues, and seems to be a cheap solution to the problem. But, as explained above, due to the filename preserving semantics, we cannot just validate based on D_CASEFOLD_LOOKUP. For that, one solution would be to invalidate the negative dentry when it is decided to turn it positive, instead of reusing it. I implemented that in the past (2018) but Al Viro made it clear we don't want to incur costs on the VFS critical path for filesystems who don't care about case-insensitiveness. Instead, this patch invalidates negative dentries in casefold directories in d_revalidate during creation lookups, iff the lookup name is not exactly what is cached. Other kinds of lookups wouldn't need this limitation. * caveats 1) Encryption Negative dentries on case-insensitive encrypted directories are also disabled. No semantic change for them is intended in this patchset; we just bypass the revalidation directly to fscrypt, for positive dentries. Encryption support is future work. 2) revalidate the cached dentry using the name under lookup Validating based on the lookup name is strange for a cache. the new semantic is implemented by d_revalidate, to stay out of the critical path of filesystems who don't care about case-insensitiveness, as much as possible. The only change is the addition of a new flavor of d_revalidate. * Tests There are a tests in place for most of the corner cases in generic/556. They mainly verify the name-preserving semantics. The invalidation when converting the directory is harder to test, because it is hard to force the invalidation of specific cached dentries that occlude a dangling invalid dentry. I tested it with forcing the positive dentries to be removed, but I'm not sure how to write an upstreamable test. It also survives fstests quick group regression testing on both ext4 and f2fs. * Performance The latency of lookups of non-existing files is obviously improved, as would be expected. The following numbers compare the execution time of 10^6 lookups of a non-existing file in a case-insensitive directory pre-populated with 100k files in ext4. Without the patch: 10.363s / 0.349s / 9.920s (real/user/sys) With the patch: 1.752s / 0.276s / 1.472s (real/user/sys) * patchset Patch 1 introduces a new flavor of d_revalidate to provide the filesystem with the name under lookup; Patch 2 introduces the new flag to signal the dentry creation context; Patch 3 introduces a libfs helper to revalidate negative dentries on case-insensitive directories; Patch 4 deals with encryption; Patch 5 cleans up the now redundant dentry operations for case-insensitive with and without encryption; Finally, Patch 6 and 7 enable support on case-insensitive directories for ext4 and f2fs, respectively. Gabriel Krisman Bertazi (7): fs: Expose name under lookup to d_revalidate hook fs: Add DCACHE_CASEFOLDED_NAME flag libfs: Validate negative dentries in case-insensitive directories libfs: Chain encryption checks after case-insensitive revalidation libfs: Merge encrypted_ci_dentry_ops and ci_dentry_ops ext4: Enable negative dentries on case-insensitive lookup f2fs: Enable negative dentries on case-insensitive lookup Documentation/filesystems/locking.rst | 3 + Documentation/filesystems/vfs.rst | 12 ++++ fs/dcache.c | 10 ++- fs/ext4/namei.c | 35 ++-------- fs/f2fs/namei.c | 25 ++----- fs/libfs.c | 93 ++++++++++++++++++--------- fs/namei.c | 23 ++++--- include/linux/dcache.h | 9 +++ 8 files changed, 116 insertions(+), 94 deletions(-)