Hi Vinícius! On Tue, Feb 13, 2024 at 01:23:55PM -0300, Vinícius Schütz Piva wrote: > Sorry for the duplicate email; tried sending to myself to doublecheck > and forget to clear the Cc. > > The getdents.2 man page details a pair syscalls: getdents() and > getdents64(), both of which are used to get the entries of a directory. > The results are populated into a structure, with the difference between > both syscalls being mostly bitwidth related. > > However, the behaviour or the 'd_off' field in both struct linux_dirent > and linux_dirent64 is wrongly documented in this man page. > > According to the current documentation, 'd_off' is used to store the > "Offset to the next linux_dirent [...] the distance from the start of > the directory to the start of the next linux_dirent." > > This value, thought, is filesystem dependent, and much of the time it > stores no such offset. > > According to readdir.3 [1] manpage: > > > The value returned in d_off is the same as would be returned by > > calling telldir(3) at the current position in the directory stream. > > Be aware that despite its type and name, the d_off field is seldom > > any kind of directory offset on modern filesystems. Applications > > should treat this field as an opaque value, making no assumptions > > about its contents; see also telldir(3). > > Of course, readdir(3) is a glibc function with no ties to > getdents(2), but it was implemented with such syscall and considering > that readdir(3) doesn't process the data from getdents(2) my belief is > that it inherited said behaviour from it [2]. telldir(3) tells a similar > story. > > On the example provided at the end of getdents.2, notable is the d_off > value of the very last entry: > > --------------- nread=120 --------------- > inode# file type d_reclen d_off d_name > 2 directory 16 12 . > 2 directory 16 24 .. > 11 directory 24 44 lost+found > 12 regular 16 56 a > 228929 directory 16 68 sub > 16353 directory 16 80 sub2 > 130817 directory 16 4096 sub3 > > which makes a very sudden jump that is obviously not where the entry is > located. > > Rerunning this same example but on a ext4 partition gives you garbage > values: > > --------------- nread=176 --------------- > inode# file type d_reclen d_off d_name > 2050 directory 24 4842312636391754590 sub2 > 2 directory 24 4844777444668968292 .. > 2051 directory 24 7251781863886579875 sub3 > 12 regular 24 7470722685224223838 a > 2049 directory 24 7653193867028490235 sub > 11 directory 32 7925945214358802294 lost+found > 2 directory 24 9223372036854775807 . > > In fact, I've had a hard time reproducing nice d_off values on ext2 too, > so what the filesystem does with d_off must have change since then. > > On tmpfs it's a count: > > --------------- nread=144 --------------- > inode# file type d_reclen d_off d_name > 1 directory 24 1 . > 1 directory 24 2 .. > 5 directory 24 3 sub3 > 4 directory 24 4 sub2 > 3 directory 24 5 sub > 2 regular 24 6 a > > I've also not been the first to notice this, as you can see from this > stackoverflow issue opened last year: > > https://stackoverflow.com/q/75119224 > > Safe to say, it's a very unreliable field. > > Below is a patch that adds a warning besides the d_off field in both > structures, plus a brief explanation on why this field can be mislea- > ding (while also directing the user towards the readdir.3 man page). > > [1] https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/man3/readdir.3 > [2] https://elixir.bootlin.com/glibc/glibc-2.39/source/sysdeps/unix/sysv/linux/readdir.c > > Signed-off-by: Vinícius Schütz Piva <vinicius.vsczpv@xxxxxxxxxxx> > --- Sorry for the late reply! Thanks! I've applied your patch, with small changes: - Put line breaks in more appropriate places, according to $ MANWIDTH=72 man 7 man-pages | sed -n '/Use semantic newlines/,/^$/p' Use semantic newlines In the source of a manual page, new sentences should be started on new lines, long sentences should be split into lines at clause breaks (commas, semicolons, colons, and so on), and long clauses should be split at phrase boundaries. This convention, sometimes known as "semantic newlines", makes it easier to see the effect of patches, which often operate at the level of individual sen‐ tences, clauses, or phrases. - Fix some typos in the commit message, and start the Subject with uppercase. Use 2 spaces after period. Put the links in a Link: field. - Bracket URIs, according to $ MANWIDTH=72 man 7 uri | sed -n '/Writing a URI/,/^$/p' Writing a URI When written, URIs should be placed inside double quotes (e.g., "http://www.kernel.org";), enclosed in angle brackets (e.g., <http://lwn.net>), or placed on a line by themselves. A warning for those who use double‐quotes: never move extraneous punctua‐ tion (such as the period ending a sentence or the comma in a list) inside a URI, since this will change the value of the URI. Instead, use angle brackets instead, or switch to a quoting sys‐ tem that never includes extraneous characters inside quotation marks. This latter system, called the ’new’ or ’logical’ quoting system by "Hart’s Rules" and the "Oxford Dictionary for Writers and Editors", is preferred practice in Great Britain and in vari‐ ous European languages. Older documents suggested inserting the prefix "URL:" just before the URI, but this form has never caught on. - In linux_dirent64, in the comment, refer to getdents(), which is in the same page, instead of referring to readdir(3) (which is already referred to in the getdents() description of d_off, above). I've temporarily applied the patch in <https://www.alejandro-colomar.es/src/alx/linux/man-pages/man-pages.git/commit/?h=contrib&id=e692a38195c0d011f73ab74589ed7f0797f82230> and will soon move it to master (tomorrow, likely). Have a lovely day! Alex > man2/getdents.2 | 12 ++++++++---- > 1 file changed, 8 insertions(+), 4 deletions(-) > > diff --git a/man2/getdents.2 b/man2/getdents.2 > index 0d4c379..3427f4b 100644 > --- a/man2/getdents.2 > +++ b/man2/getdents.2 > @@ -67,7 +67,7 @@ structure is declared as follows: > .EX > struct linux_dirent { > unsigned long d_ino; /* Inode number */ > - unsigned long d_off; /* Offset to next \fIlinux_dirent\fP */ > + unsigned long d_off; /* Not an offset; see below */ > unsigned short d_reclen; /* Length of this \fIlinux_dirent\fP */ > char d_name[]; /* Filename (null\-terminated) */ > /* length is actually (d_reclen \- 2 \- > @@ -84,8 +84,12 @@ struct linux_dirent { > .I d_ino > is an inode number. > .I d_off > -is the distance from the start of the directory to the start of the next > -.IR linux_dirent . > +is a filesystem specific value with no specific meaning to userspace, > +though on older filesystems it used to be the distance from the start > +of the directory to the start of the next > +.IR linux_dirent ; > +see > +.BR readdir (3) . > .I d_reclen > is the size of this entire > .IR linux_dirent . > @@ -167,7 +171,7 @@ structures of the following type: > .EX > struct linux_dirent64 { > ino64_t d_ino; /* 64\-bit inode number */ > - off64_t d_off; /* 64\-bit offset to next structure */ > + off64_t d_off; /* Not an offset; see readdir(3) */ > unsigned short d_reclen; /* Size of this dirent */ > unsigned char d_type; /* File type */ > char d_name[]; /* Filename (null\-terminated) */ > -- > 2.39.2 > -- <https://www.alejandro-colomar.es/> Looking for a remote C programming job at the moment.
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