Re: [PATCH RFC 1/3] symlink.7: document magic-links more completely
flat view
From: Jann Horn <jannh@google.com>
Date: 2019-10-07 16:37:10
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linux-man, lkml
On Thu, Oct 3, 2019 at 4:56 PM Aleksa Sarai [off-list ref] wrote:
Traditionally, magic-links have not been a well-understood topic in Linux. Given the new changes in their semantics (related to the link mode of trailing magic-links), it seems like a good opportunity to shine more light on magic-links and their semantics.
[...]
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+++ b/man7/symlink.7@@ -84,6 +84,25 @@ as they are implemented on Linux and other systems, are outlined here. It is important that site-local applications also conform to these rules, so that the user interface can be as consistent as possible. +.SS Magic-links +There is a special class of symlink-like objects known as "magic-links" which
I think names like that normally aren't hypenated in english, and instead of "magic-links", it'd be "magic links"? Just like how you wouldn't write "symbolic-link", but "symbolic link". But this is bikeshedding, and if you disagree, feel free to ignore this comment.
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+can be found in certain pseudo-filesystems such as +.BR proc (5) +(examples include +.IR /proc/[pid]/exe " and " /proc/[pid]/fd/* .) +Unlike normal symlinks, magic-links are not resolved through
nit: AFAICS symlinks are always referred to as "symbolic links" throughout the manpages.
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+pathname-expansion, but instead act as direct references to the kernel's own +representation of a file handle. As such, these magic-links allow users to +access files which cannot be referenced with normal paths (such as unlinked +files still referenced by a running program.)
Could maybe add "and files in different mount namespaces" as another example here; at least for me, that's the main usecases for /proc/*/root. [...]
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+However, magic-links do not follow this rule. They can have a non-0777 mode, +which is used for permission checks when the final +component of an +.BR open (2)'s
Maybe leave out the "open" part, since the same restriction has to also apply to other syscalls operating on files, like truncate() and so on?
+path is a magic-link (see +.BR path_resolution (7).)