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Message-ID: <87a5ut7k62.fsf@suse.de> Date: Mon, 14 Aug 2023 10:50:13 -0400 From: Gabriel Krisman Bertazi <krisman@...e.de> To: Eric Biggers <ebiggers@...nel.org> Cc: viro@...iv.linux.org.uk, brauner@...nel.org, tytso@....edu, jaegeuk@...nel.org, linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net, Gabriel Krisman Bertazi <krisman@...labora.com> Subject: Re: [PATCH v5 06/10] libfs: Validate negative dentries in case-insensitive directories Eric Biggers <ebiggers@...nel.org> writes: > On Fri, Aug 11, 2023 at 08:41:42PM -0400, Gabriel Krisman Bertazi wrote: >> + /* >> + * Filesystems will call into d_revalidate without setting >> + * LOOKUP_ flags even for file creation (see lookup_one* >> + * variants). Reject negative dentries in this case, since we >> + * can't know for sure it won't be used for creation. >> + */ >> + if (!flags) >> + return 0; >> + >> + /* >> + * 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. >> + */ >> + if (flags & (LOOKUP_CREATE | LOOKUP_RENAME_TARGET)) { >> + /* >> + * ->d_name won't change from under us in the creation >> + * path only, since d_revalidate during creation and >> + * renames is always called with the parent inode >> + * locked. It isn't the case for all lookup callpaths, >> + * so ->d_name must not be touched outside >> + * (LOOKUP_CREATE|LOOKUP_RENAME_TARGET) context. >> + */ >> + if (dentry->d_name.len != name->len || >> + memcmp(dentry->d_name.name, name->name, name->len)) >> + return 0; >> + } > > This is still really confusing to me. Can you consider the below? The code is > the same except for the reordering, but the explanation is reworked to be much > clearer (IMO). Anything I am misunderstanding? > > /* > * 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. > * > * LOOKUP_CREATE or LOOKUP_RENAME_TARGET cover most creations. In these > * cases, ->d_name is stable and can be compared to 'name' without > * taking ->d_lock because the caller holds dir->i_rwsem for write. > * (This is because the directory lock blocks the dentry from being > * concurrently instantiated, and negative dentries are never moved.) > * > * All other creations actually use flags==0. These come from the edge > * case of filesystems calling functions like lookup_one() that do a > * lookup without setting the lookup flags at all. Such lookups might > * or might not be for creation, and if not don't guarantee stable > * ->d_name. Therefore, invalidate all negative dentries when flags==0. > */ > if (flags & (LOOKUP_CREATE | LOOKUP_RENAME_TARGET)) { > if (dentry->d_name.len != name->len || > memcmp(dentry->d_name.name, name->name, name->len)) > return 0; > } > if (!flags) > return 0; I don't see it as particularly better or less confusing than the original. but I also don't mind taking it into the next iteration. -- Gabriel Krisman Bertazi
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