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A magic entry registered with 'D' and the same entry without it, to pin
down what the flag decides and what it leaves alone:
- the entry reports itself disabled and nothing dispatches until '1' is
written to it
- without 'D' it dispatches straight away
- 'D' is not read back among the entry's flags
- enabling and disabling afterwards works as it does for any entry
- 'D' composes with the flags that shape the invocation
- '-1' to the status file removes a staged entry like any other
- a file handle held across a removal cannot resurrect the entry
Put the entry write and read-back helpers into binfmt_misc_common.h.
The bpf suite will need them as well.
Link: https://patch.msgid.link/20260730-work-binfmt_misc-preopen-v1-3-4a0b0da71f16@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
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binfmt_flag_supported() returns 0 when the flag is supported and -1
when it is not, so every caller reads backwards:
if (binfmt_flag_supported('T'))
SKIP(return, "kernel without the 'T' flag");
Make it return a bool and flip the callers. errno from a failed probe
is still set for callers that check it.
Link: https://patch.msgid.link/20260730-work-binfmt_misc-preopen-v1-2-4a0b0da71f16@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
Reviewed-by: Farid Zakaria <farid.m.zakaria@gmail.com>
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Exercise the 'L' flag end to end. The payload runs as the main image
with a copy of the system loader substituted for its PT_INTERP, and
asserts the native identity from inside:
- argv exactly as the caller built it
- no AT_EXECFD
- AT_FLAGS clear
- AT_BASE set but outside its own image
- AT_PHDR/AT_ENTRY inside it
- /proc/self/{exe,comm,stat} and AT_EXECFN all describing the binary
- ETXTBSY on the running binary
- the substituted loader visible in /proc/self/maps under its real path
Magic matching pokes a marker into the ELF header's e_ident padding
(EI_PAD, offset 9), which sits inside the match window and is ignored by
kernel and loader alike. the same binary is also matched by extension.
Two cases cover the paths where the substitution does not happen. A '#!'
file that matched an 'L' entry is claimed by binfmt_script rather than by
binfmt_elf, so the staged substitute has to be released when the
interpreter replaces the file; the test opens the loader for writing
afterwards, which fails with ETXTBSY if the write denial was leaked
instead. A relative interpreter path is rejected at registration for both
'L' and 'C', neither of which may resolve one against the working
directory of whoever runs the binary.
The bpf-side BPF_BINPRM_LOADER path shares all machinery past the flag
mapping. A harness case for it can join the bpf runtime coverage of
the transparent series.
Link: https://patch.msgid.link/20260721-work-bpf-binfmt_misc-ptinterp-v2-20-e57866e4ae0f@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
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Verify the identity a transparent dispatch constructs, from both
activation paths.
- binfmt_misc_transparent: registers a magic entry with the static 'T'
flag and execs a matched binary with arguments.
- binfmt_misc_bpf: a handler whose load program sets
BPF_BINPRM_TRANSPARENT.
Both dispatch to a shared asserting interpreter that runs in place of
the binary and checks the contract from the inside:
- AT_FLAGS carries AT_FLAGS_TRANSPARENT_INTERP
- AT_EXECFD refers to the very inode of the binary
- /proc/self/exe resolves to the binary
- argv and /proc/self/cmdline are exactly what the caller passed with
nothing spliced in
- comm is the binary's basename
- the binary is write-denied while it runs
The static test also validates the registration. 'T' combined with 'P'
must be rejected. A kernel that does not know 'T' turns the test into a
skip. The asserting interpreter and the static test build without the
bpf toolchain so the core transparent semantics stay covered on systems
where the bpf cases are skipped.
The flag support probe, the canonical payload argv with the
run_payload() helper that execs it, and the identity assertions (exe
link, comm, write denial) live in binfmt_misc_common.h; the loader
substitution test reuses all of them.
Link: https://patch.msgid.link/20260721-work-bpf-binfmt_misc-ptinterp-v2-13-e57866e4ae0f@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
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The test reports its own pass and fail lines, returns a bare 4 for
KSFT_SKIP and runs both cases in one process, so a failure in the first
takes the second with it. It also open-codes the register, unregister,
file-copy and mount helpers that the tests for the upcoming transparent
and loader dispatch modes need again.
Convert it to the kselftest harness: a fixture for the common setup and
teardown, one TEST_F per case so each is reported and isolated
separately, and SKIP() for the root, BTF and binfmt_misc preconditions.
Move the helpers to a shared header on the way, with the register
helper preserving the write's errno so a caller can tell a rejected
flag combination (EINVAL) from a kernel that does not know the flag at
all. The synthetic ELF header gains an e_machine argument and uses the
elf.h constants instead of open-coded numbers.
The fixture no longer mounts bpffs. The handler is attached with
bpf_map__attach_struct_ops() and nothing is ever pinned, the mount was
carried along from a bpftool-based draft. The bpf objects are compiled
with -DBPF_NO_KFUNC_PROTOTYPES - the guard bpftool emits for exactly
this - instead of sed'ing the prototypes out of the generated
vmlinux.h. And the config fragment records the options the binfmt_misc
tests need so a merge-config kernel can run them.
No change in what is tested.
Link: https://patch.msgid.link/20260721-work-bpf-binfmt_misc-ptinterp-v2-7-e57866e4ae0f@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
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