I agree with you, I also enjoy reading it, and admire the method as well as the result in this case.
FUSE is immensely useful, also. Its the front-/back- door to a lot of things. There's only a few steps left to a tiny crypto-stack, hosted on top of it ..
The glibc test suite contains a few tiny FUSE file systems. For example, there is one that just happens to contain every file that mkstemp attempts to create, for a test that exercises the O_CREAT|O_EXCL failure path. Like the Rust fuser crate, it uses the kernel API directly. The tests are slightly brittle because sometimes we encounter a new LSM that triggers unexpected file system operations, but it's not too bad overall.
What I found funny when I discovered it is that you can create a thread in the same process that provides the FUSE file system implementation for this very process. It makes it much easier to write certain tests, especially debugging. We had to teach valgrind that more system calls effectively perform callbacks into the same process, but fortunately valgrind already had a FUSE_COMPATIBLE_MAY_BLOCK mechanism for that.
Everything we need is already upstream. I think the changes were:
commit 0690dc39644d15fc89813419ffcdf9754b098260
Author: Mark Wielaard <mark@klomp.org>
Date: Sun Sep 22 23:24:34 2024 +0200
Implement /proc/self/exe readlink[at] fallback in POST handler
Calling the readlink[at] syscall directly from the PRE handler defeats
the FUSE_COMPATIBLE_MAY_BLOCK (SfMayBlock) flag. Add a POST handler
that only explicitly calls the readlink[at] handler for the
/proc/self/exe fallback (this should be fine unless /proc is also
implemented as fuse in this process).
Adjust readlink[at] GENX_ and LINX_ syswrap macros to GENXY and LINXY.
https://bugs.kde.org/show_bug.cgi?id=493507
commit ddf397c024c80382f7a2f3a0d46d58fb839eef96
Author: Mark Wielaard <mark@klomp.org>
Date: Sat Sep 21 22:27:24 2024 +0200
Add missing FUSE_COMPATIBLE_MAY_BLOCKs
Various syscalls (in particular "at" variants) PRE handlers were
missing a FUSE_COMPATIBLE_MAY_BLOCK statement.
Add it to the generic PRE handlers of access and statfs64. And the
linux PRE handlers of mknodat, fchownat, futimesat, utimensat,
utimensat_time64, renameat, renameat2, readlinkat, fchmodat,
fchmodat2, faccessat and faccessat2.
https://bugs.kde.org/show_bug.cgi?id=493454
11 comments
[ 2.5 ms ] story [ 39.2 ms ] threadFUSE is immensely useful, also. Its the front-/back- door to a lot of things. There's only a few steps left to a tiny crypto-stack, hosted on top of it ..
What I found funny when I discovered it is that you can create a thread in the same process that provides the FUSE file system implementation for this very process. It makes it much easier to write certain tests, especially debugging. We had to teach valgrind that more system calls effectively perform callbacks into the same process, but fortunately valgrind already had a FUSE_COMPATIBLE_MAY_BLOCK mechanism for that.