From 034dd340b08be1f2f0477ad16131d609f9dbd53c Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Sun, 30 Aug 2026 09:22:00 -0700 Subject: Merge tag 'trace-v7.3-2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace Pull tracing fixes from Steven Rostedt: - Fix error output of boot instance creation failure Currently if a boot instance creation fails, instead of printing out the name of the instance that failed, it prints "(null)". That is because it prints "cur_str" that had already been processed by strsep(). Print the saved name instead. While at it, print the error code of the failure. - Fix use-after-free for same named historgrams Histograms can be named so that they can be used in multiple events. But if the named histogram has a variable attached, the second event that uses the named histogram which duplicates it and needs to free the original after duplication leaves the old variable in place and still visible. If another histogram uses than variable, it will use the stale one which will try to reference the freed duplicate histogram and crash the kernel. Free the duplicate variables along with the duplicated histogram data. - Check return value of kthread_run() in event self test The events self tests uses a kthread for testing but does not check if it succeeded in creating a kthread. If the kthread creation were to fail, the code will still try to call kthread_stop() on the error returned. - Fix race between reading trace_pipe and updating subbuffer size If a user is reading the trace_pipe file at the same time they update the ring buffer sub-buffer size, can cause the trace_pipe read to read stale data. Add trace_access_lock() around updating the ring buffer sub-buffer size. - Fix eventfs_inode on failure path in creation of the events directory In the creation of the "events" directory, if after allocating the eventfs_inode a failure is detected, it calls cleanup_ei() which calls free_ei(). The free_ei() will test if eventfs_inode being freed has no children. It is a bug if it does. But on the failure case of the creation of the "events" directory, the children lists have not yet been initialized and the free will trigger a warning because list_empty() on an uninitialized list returns false. Move the initialization into init_ei() where it makes more sense and makes sure that a created eventfs_inode has its lists initialized upon creation. - Check return value of kthread_run() in ftrace direct sample code The sample code that shows how to use the ftrace direct calls does not test the return of kthread_run() to see if it succeeds. Return a failure if the kthread_run() doesn't succeed. - Clear user events state on fork in case of alloc failure On fork, the child gets a pointer to the parent's user events state. It makes a copy of it then updates the child's pointer to it. But if the allocation fails, the duplication function leaves the child with a pointer to its parent's descriptor. When the child cleans up its data, it will free the parent's descriptor while the parent is still using it. In the duplication function, set the child's user_event_mm to NULL before testing if the allocation succeeded, and when it exits it will not free the parent's descriptor. - Fix retry exhaustion in simple ring buffer reader swap simple_ring_buffer_swap_reader_page() starts with retry set to 8 and post-decrements it only after a failed link replacement. On the final attempt, a successful replacement leaves retry at zero, while a failed replacement leaves it at -1. But the check for success expects the retry value to be non-zero and exits with an error on zero. This is the opposite result. Fix it. - Fail nicely when the remote swap_reader_page() returns an error Currently, if the swap_reader_page() of a remote buffer fails, it triggers a WARN_ON_ONCE() and continues normally. Instead, have it exit with an error and a pr_warn() print instead of a full WARNING. * tag 'trace-v7.3-2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace: ring-buffer: Stop remote reader update when page swap fails tracing: Fix retry exhaustion in simple ring buffer reader swap tracing/user_events: Clear copied tracing state before fork duplication samples/ftrace: Fix kthread_stop() on ERR_PTR in ftrace-direct-multi-modify samples/ftrace: Fix kthread_stop() on ERR_PTR in ftrace-direct-modify eventfs: Initialize ei->children and ei->list in init_ei() tracing: Fix use-after-free in trace_pipe read on sub-buffer order change tracing: Fix crash passing ERR_PTR to kthread_stop() tracing: Fix use-after-free with same-name named triggers tracing: Fix logged instance name on creation failure --- Documentation/admin-guide/binfmt-misc.rst | 391 ++++++++++++++++++++++++++++++ 1 file changed, 391 insertions(+) create mode 100644 Documentation/admin-guide/binfmt-misc.rst (limited to 'Documentation/admin-guide/binfmt-misc.rst') diff --git a/Documentation/admin-guide/binfmt-misc.rst b/Documentation/admin-guide/binfmt-misc.rst new file mode 100644 index 000000000..d26b63a27 --- /dev/null +++ b/Documentation/admin-guide/binfmt-misc.rst @@ -0,0 +1,391 @@ +Kernel Support for miscellaneous Binary Formats (binfmt_misc) +============================================================= + +This Kernel feature allows you to invoke almost (for restrictions see below) +every program by simply typing its name in the shell. +This includes for example compiled Java(TM), Python or Emacs programs. + +To achieve this you must tell binfmt_misc which interpreter has to be invoked +with which binary. Binfmt_misc recognises the binary-type by matching some bytes +at the beginning of the file with a magic byte sequence (masking out specified +bits) you have supplied. Binfmt_misc can also recognise a filename extension +aka ``.com`` or ``.exe``. + +First you must mount binfmt_misc:: + + mount binfmt_misc -t binfmt_misc /proc/sys/fs/binfmt_misc + +To actually register a new binary type, you have to set up a string looking like +``:name:type:offset:magic:mask:interpreter:flags`` (where you can choose the +``:`` upon your needs) and echo it to ``/proc/sys/fs/binfmt_misc/register``. + +Here is what the fields mean: + +- ``name`` + is an identifier string. A new /proc file will be created with this + name below ``/proc/sys/fs/binfmt_misc``; cannot contain slashes ``/`` for + obvious reasons. +- ``type`` + is the type of recognition. Give ``M`` for magic, ``E`` for extension and + ``B`` for a bpf-backed handler (see below). +- ``offset`` + is the offset of the magic/mask in the file, counted in bytes. This + defaults to 0 if you omit it (i.e. you write ``:name:type::magic...``). + Ignored when using filename extension matching. +- ``magic`` + is the byte sequence binfmt_misc is matching for. The magic string + may contain hex-encoded characters like ``\x0a`` or ``\xA4``. Note that you + must escape any NUL bytes; parsing halts at the first one. In a shell + environment you might have to write ``\\x0a`` to prevent the shell from + eating your ``\``. + If you chose filename extension matching, this is the extension to be + recognised (without the ``.``, the ``\x0a`` specials are not allowed). + Extension matching is case sensitive, and slashes ``/`` are not allowed! +- ``mask`` + is an (optional, defaults to all 0xff) mask. You can mask out some + bits from matching by supplying a string like magic and as long as magic. + The mask is anded with the byte sequence of the file. Note that you must + escape any NUL bytes; parsing halts at the first one. Ignored when using + filename extension matching. +- ``interpreter`` + is the program that should be invoked with the binary as first + argument (specify the full path). For ``B`` entries this field + carries the name of the bpf handler instead (see below). +- ``flags`` + is an optional field that controls several aspects of the invocation + of the interpreter. It is a string of capital letters, each controls a + certain aspect. The following flags are supported: + + ``P`` - preserve-argv[0] + Legacy behavior of binfmt_misc is to overwrite + the original argv[0] with the full path to the binary. When this + flag is included, binfmt_misc will add an argument to the argument + vector for this purpose, thus preserving the original ``argv[0]``. + e.g. If your interp is set to ``/bin/foo`` and you run ``blah`` + (which is in ``/usr/local/bin``), then the kernel will execute + ``/bin/foo`` with ``argv[]`` set to ``["/bin/foo", "/usr/local/bin/blah", "blah"]``. The interp has to be aware of this so it can + execute ``/usr/local/bin/blah`` + with ``argv[]`` set to ``["blah"]``. + ``O`` - open-binary + Legacy behavior of binfmt_misc is to pass the full path + of the binary to the interpreter as an argument. When this flag is + included, binfmt_misc will open the file for reading and pass its + descriptor into the auxilary vector with the key "AT_EXECFD", thus + allowing the interpreter to execute non-readable binaries. This + feature should be used with care - the interpreter has to be trusted + not to emit the contents of the non-readable binary. + ``C`` - credentials + Currently, the behavior of binfmt_misc is to calculate + the credentials and security token of the new process according to + the interpreter. When this flag is included, these attributes are + calculated according to the binary. It also implies the ``O`` flag. + This feature should be used with care as the interpreter + will run with root permissions when a setuid binary owned by root + is run with binfmt_misc. + ``F`` - fix binary + The usual behaviour of binfmt_misc is to spawn the + binary lazily when the misc format file is invoked. However, + this doesn't work very well in the face of mount namespaces and + changeroots, so the ``F`` mode opens the binary as soon as the + emulation is installed and uses the opened image to spawn the + emulator, meaning it is always available once installed, + regardless of how the environment changes. + ``T`` - transparent + Run the interpreter transparently. The binary is handed to + the interpreter through ``AT_EXECFD`` (``T`` implies ``O``), + the argument vector is left exactly as the caller built it + and the kernel labels ``/proc/pid/exe`` with the binary + instead of the interpreter. The interpreter has to load the + binary from ``AT_EXECFD`` and follow the + ``AT_FLAGS_TRANSPARENT_INTERP`` contract. Combining ``T`` + with ``P`` is rejected: transparency preserves the whole + argument vector, argv[0] included. + ``L`` - loader substitution + Do not run the interpreter on the binary at all: load the + binary itself as a fully native exec and substitute the + interpreter for the loader named in the binary's + ``PT_INTERP``. See the "Loader substitution" section + below. ``L`` rejects ``T``, ``P``, ``O`` and ``C``; + ``F`` composes. + ``D`` - registered disabled + The entry is created disabled instead of being matchable at + once, and has to be enabled by writing ``1`` to its file + before it dispatches anything. This splits a registration + into creating the entry and activating it, leaving room to + configure it in between - which is what a ``B`` entry that + binds interpreters needs; see the bpf section below. The flag + is spent on the registration and is not read back: what an + entry file reports afterwards is whether it is enabled. + + +There are some restrictions: + + - the whole register string may not exceed 1920 characters + - the magic must reside in the first 128 bytes of the file, i.e. + offset+size(magic) has to be less than 128 + - the interpreter string may not exceed 127 characters + - an interpreter used with ``C`` or ``L`` but without ``F`` has to be + named by an absolute path. It is opened when the binary is executed, so + a relative one would be resolved against the working directory of + whoever runs the binary + - the amount of pre-opened interpreters by ``F``, or bound to a ``B`` entry + is limited by the ``/proc/sys/user/max_binfmt_misc_interpreters`` sysctl. A + registration past the limit is refused with ``-ENOSPC``. This limits an + unprivileged namespace pinning files. A nested namespace can raise only its + own limit and every ancestor is charged too + + +To use binfmt_misc you have to mount it first. You can mount it with +``mount -t binfmt_misc none /proc/sys/fs/binfmt_misc`` command, or you can add +a line ``none /proc/sys/fs/binfmt_misc binfmt_misc defaults 0 0`` to your +``/etc/fstab`` so it auto mounts on boot. + +You may want to add the binary formats in one of your ``/etc/rc`` scripts during +boot-up. Read the manual of your init program to figure out how to do this +right. + +Think about the order of adding entries! Later added entries are matched first! + + +A few examples (assumed you are in ``/proc/sys/fs/binfmt_misc``): + +- enable support for em86 (like binfmt_em86, for Alpha AXP only):: + + echo ':i386:M::\x7fELF\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\x03:\xff\xff\xff\xff\xff\xfe\xfe\xff\xff\xff\xff\xff\xff\xff\xff\xff\xfb\xff\xff:/bin/em86:' > register + echo ':i486:M::\x7fELF\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\x06:\xff\xff\xff\xff\xff\xfe\xfe\xff\xff\xff\xff\xff\xff\xff\xff\xff\xfb\xff\xff:/bin/em86:' > register + +- enable support for packed DOS applications (pre-configured dosemu hdimages):: + + echo ':DEXE:M::\x0eDEX::/usr/bin/dosexec:' > register + +- enable support for Windows executables using wine:: + + echo ':DOSWin:M::MZ::/usr/local/bin/wine:' > register + +For java support see Documentation/admin-guide/java.rst + + +You can enable/disable binfmt_misc or one binary type by echoing 0 (to disable) +or 1 (to enable) to ``/proc/sys/fs/binfmt_misc/status`` or +``/proc/.../the_name``. +Catting the file tells you the current status of ``binfmt_misc/the_entry``. + +You can remove one entry or all entries by echoing -1 to ``/proc/.../the_name`` +or ``/proc/sys/fs/binfmt_misc/status``. A single entry can also be removed +by simply unlinking (``rm``) ``/proc/.../the_name``. + + +bpf-backed handlers +------------------- + +With ``CONFIG_BINFMT_MISC_BPF`` both the matching and the interpreter +selection can be delegated to bpf programs. A handler is an instance of the +``binfmt_misc_ops`` struct_ops with a ``match`` and a ``load`` program and a +``name``. Once the struct_ops map is registered the handler can be activated +with a ``B`` entry that references it by name in the ``interpreter`` field +and carries neither offset, magic, nor mask:: + + echo ':qemu:B::::my_handler:' > register + +Both programs receive the ``linux_binprm`` of the binary and both can +sleep. The ``match`` program decides whether the handler applies: it is +consulted during the entry walk exactly like magic and extension matching, +in the same registration order with the same first-match-wins semantics. +Unlike static matching it is not limited to the prefetched first bytes of +the file in ``bprm->buf``: it can read the file, e.g. to parse ELF program +headers whose data sits at arbitrary offsets. It only decides, though: the +selection kfuncs below are rejected in it. The ``load`` program of the +matched handler then selects the interpreter: it can equally read the file +and derive the interpreter from the binary's location. It selects the +interpreter by calling the ``bpf_binprm_set_interp()`` kfunc with an +absolute path and returning ``0``. A match is committed: a failing +``load`` fails the exec with its error instead of falling through to later +entries; ``-ENOEXEC`` lets the remaining binary formats have a go. A path +selected this way is opened with the credentials of the task doing the +exec, exactly as a statically registered interpreter without ``F`` would +be. + +An entry can instead bind the interpreters its handler may use, so that no +path is resolved at exec time at all. An entry registered with ``D`` is not +matchable yet, which is what leaves it open to being given them, one +``+name path`` write at a time:: + + echo ':qemu:B::::my_handler:D' > register + echo '+aarch64 /usr/bin/qemu-aarch64' > qemu + echo '+arm /usr/bin/qemu-arm' > qemu + echo 1 > qemu + +Each path is opened during its write, in the writing process's context and +with the credentials the entry file was opened with, exactly the way ``F`` +pre-opens a static entry's interpreter; the paths must be absolute. The +path is everything past the first space, so there is nothing it cannot +express, and no interpreter has to fit in a register string. An entry +binds at most 100 interpreters, and each one is charged against +``max_binfmt_misc_interpreters`` like any other binding. A write past either +limit is refused with ``-ENOSPC``. + +The ``load`` program then selects one per exec by name with the +``bpf_binprm_select_interp()`` kfunc, and every exec runs a clone of the +file that was opened. The path decides which file is bound and nothing +else: it is not resolved again, in any namespace, so what it holds later - +or what it holds in the namespace of whoever runs the binary - no longer +decides anything. + +Enabling the entry ends this. Its interpreters are read at exec time with +nothing but a reference held on the entry, so an entry that has ever been +matchable can never have its set changed again: the first ``1`` seals it, +from then on ``+`` is refused with ``-EBUSY``, and an entry registered +without ``D`` is sealed from the start. Binding a name twice is refused +with ``-EEXIST``. + +Selection is by name so that the configuration and the program need not +agree on an order, and so that a handler is not tied to where a distribution +puts its interpreters. A name is a single word of printable ASCII, at most +32 characters; a name the entry did not bind gives the program ``-ENOENT``, +which it can act on or return. The interpreter runs under the path it was +registered under, and the entry reports what it bound:: + + $ cat /proc/sys/fs/binfmt_misc/qemu + enabled + bpf my_handler + bpf-interpreter aarch64 /usr/bin/qemu-aarch64 + bpf-interpreter arm /usr/bin/qemu-arm + flags: + +The path reported is the one the interpreter was bound under, which named +the file at that moment; it is not re-resolved, so it is a record of what +was bound rather than a promise about what that path holds now. + +The ``load`` program can also pass a single argument to the interpreter with +the ``bpf_binprm_set_interp_arg()`` kfunc. It is inserted between the +interpreter and the binary, exactly like the optional argument of a ``#!`` +interpreter line, e.g. for a handler that resolves ``$ORIGIN`` in a script's +``#!`` path and needs to preserve the argument that followed it. + +The invocation flags a static entry fixes at registration - ``P``, ``C``, +``O``, ``T`` and ``L`` - are per-exec choices for a bpf handler, made by the +``load`` program with the ``bpf_binprm_set_flags()`` kfunc, so a single +handler can decide them differently for each binary it handles: + +- ``BPF_BINPRM_PRESERVE_ARGV0`` keeps the caller's ``argv[0]`` (the ``P`` + flag). +- ``BPF_BINPRM_CREDENTIALS`` computes credentials from the binary (the ``C`` + flag), bounded to user namespaces that map the binary's owner just like + any other setuid exec. +- ``BPF_BINPRM_EXECFD`` opens the binary on the interpreter's behalf and + passes it through the ``AT_EXECFD`` aux vector entry (the ``O`` flag), so + the interpreter can run binaries it could not open by path. +- ``BPF_BINPRM_TRANSPARENT`` runs the interpreter transparently (the ``T`` + flag): the binary is handed over through ``AT_EXECFD`` as + with ``BPF_BINPRM_EXECFD``, but the argument vector is also left as the + caller passed it. An interpreter that loads the binary from ``AT_EXECFD`` + then appears in ``argv[0]`` and ``/proc/pid/cmdline`` as a direct + execution of the binary. ``BPF_BINPRM_PRESERVE_ARGV0`` and a staged + interpreter argument are rejected in combination with it, just as ``P`` + is with ``T``. It also lets a handler + run a binary passed as an inaccessible ``O_CLOEXEC`` file descriptor to + ``execveat()``, which a path-splicing dispatch cannot: the interpreter + has no path by which to open it. +- ``BPF_BINPRM_LOADER`` substitutes the interpreter for the binary's + ``PT_INTERP`` and runs the binary as a fully native exec (the ``L`` + flag). It excludes the other flags and a staged interpreter argument. + +Because these are program choices, a ``B`` entry carries no invocation +flags in the register string; ``F`` has none to spell for it either, since +the interpreters it binds already pre-open what ``F`` would. The +registration directive ``D`` is the exception: it decides how the entry +starts out, not how the interpreter is invoked. + +A handler is looked up only in the user namespace the struct_ops map was +registered in. Handlers are not inherited, so an entry can only reference a +handler registered in the same user namespace as its binfmt_misc instance. +The entry keeps the handler alive; deleting the struct_ops map only prevents +new activations. + + +Transparent interpreters +------------------------ + +With the ``T`` flag or ``BPF_BINPRM_TRANSPARENT`` the dispatch is invisible +to the resulting process. The argument vector is left exactly as the caller +built it. The binary is passed through ``AT_EXECFD``. The kernel also labels +``/proc/pid/exe`` correctly. The binary's file is write-denied while the +process runs and the interpreter's is not, exactly as if the binary had been +executed directly. A transparent entry does not change how credentials are +derived. As +with any other entry, set*id bits of the binary are only honored with ``C`` (or +``BPF_BINPRM_CREDENTIALS``). + +The interpreter has to be built for this contract. The kernel announces it +with ``AT_FLAGS_TRANSPARENT_INTERP`` in the ``AT_FLAGS`` aux vector entry +next to ``AT_EXECFD``. The argument vector belongs entirely to the program, +nothing was spliced in, so the interpreter doesn't consume arguments and +simply loads the program from the descriptor. The bit is also the loader's +license to finish the identity. After mapping the program it may retarget the +``AT_PHDR``/``AT_ENTRY``/``AT_BASE`` entries of ``/proc/pid/auxv`` and the +code/data statistics markers via one ``PR_SET_MM_MAP`` which completes +what attaching debuggers observe. What remains visibly different from a +direct execution is the address space layout. The interpreter occupies +the main-image position and the program lives in the mmap region. + + +Loader substitution +------------------- + +The ``L`` flag turns the execution model around. Instead of running the +registered interpreter with the binary as its payload the kernel loads +the matched binary itself as the main image and substitutes the registered +interpreter for the loader named in the binary's ``PT_INTERP``. + +Because the exec is native, there is no dispatch identity to +reconstruct and no contract the substitute has to implement. A stock +dynamic loader works unchanged. The argument vector is untouched, +credentials and ``AT_SECURE`` derive from the binary, there is no +``AT_EXECFD`` and no marker in the aux vector, the binary sits in the +main-image slot with the native brk placement so ``/proc/pid/maps``, +core dumps and perf mmap records have the native shape, and the +identity is already complete when ``PTRACE_EVENT_EXEC`` stops the +tracee. So launching under a debugger works, not just attaching. ``L`` +entries are for ELF binaries of a native architecture. Foreign-arch +emulation and non-ELF payloads remain the domain of the classic and +transparent modes. + +The override applies when the format that finally claims the file is +ELF with a ``PT_INTERP``. A matched binary without one or an +interpreter-less ``ET_DYN`` drops the override and runs natively. A file +claimed by another format - a ``#!`` script, say - is handled by that +format as if the entry had not matched. ``L`` is therefore not an +enforcement mechanism: it decides how a binary that asks for a loader is +run, it does not guarantee that everything matching the entry runs under +the substitute. A format that cannot consume the override at all instead +refuses the exec with ``ENOEXEC`` before the point of no return. + +A wrong-architecture ELF fails the whole exec with ``ENOEXEC`` exactly +as if no entry had matched. A substitute that is not ELF of the right +architecture fails with ``ELIBBAD``. The usual ``PT_INTERP`` sanity +checks on the binary still apply. But the segment's content is otherwise +irrelevant. + +``L`` rejects the classic-dispatch flags ``T``, ``P``, ``O`` and ``C`` +at registration. ``F`` composes and is valuable: with it the substitute +is opened at registration time, so later mount namespace or path changes +cannot redirect it. Without it the substitute is opened when the binary +is executed, and the path is resolved in the mount namespace and root of +whoever runs the binary, which is why it has to be absolute. As with +``C``, register only trusted interpreters. The substituted loader runs +with credentials derived from the binary. + + +Hints +----- + +If you want to pass special arguments to your interpreter, you can +write a wrapper script for it. +See :doc:`Documentation/admin-guide/java.rst <./java>` for an example. + +Your interpreter should NOT look in the PATH for the filename; the kernel +passes it the full filename (or the file descriptor) to use. Using ``$PATH`` can +cause unexpected behaviour and can be a security hazard. + + +Richard Günther -- cgit v1.3.1