static_call: Don't make __static_call_return0 static

System.map shows that vmlinux contains several instances of
__static_call_return0():

c0004fc0 t __static_call_return0
c0011518 t __static_call_return0
c00d8160 t __static_call_return0

arch_static_call_transform() uses the middle one to check whether we are
setting a call to __static_call_return0 or not:

c0011520 <arch_static_call_transform>:
c0011520: 3d 20 c0 01 lis r9,-16383 <== r9 = 0xc001 << 16
c0011524: 39 29 15 18 addi r9,r9,5400 <== r9 += 0x1518
c0011528: 7c 05 48 00 cmpw r5,r9 <== r9 has value 0xc0011518 here

So if static_call_update() is called with one of the other instances of
__static_call_return0(), arch_static_call_transform() won't recognise it.

In order to work properly, global single instance of __static_call_return0() is required.

Fixes: 3f2a8fc4b15d ("static_call/x86: Add __static_call_return0()")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lkml.kernel.org/r/30821468a0e7d28251954b578e5051dc09300d04.1647258493.git.christophe.leroy@csgroup.eu

KVM: x86: allow defining return-0 static calls

A few vendor callbacks are only used by VMX, but they return an integer
or bool value. Introduce KVM_X86_OP_OPTIONAL_RET0 for them: if a func is
NULL in struct kvm_x86_ops, it will be changed to __static_call_return0
when updating static calls.

Reviewed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

static_call: Fix static_call_text_reserved() vs __init

It turns out that static_call_text_reserved() was reporting __init
text as being reserved past the time when the __init text was freed
and re-used.

This is mostly harmless and will at worst result in refusing a kprobe.

Fixes: 6333e8f73b83 ("static_call: Avoid kprobes on inline static_call()s")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Link: https://lore.kernel.org/r/20210628113045.106211657@infradead.org

static_call: Fix unused variable warn w/o MODULE

Here is the warning converted as error and reported by GCC:

kernel/static_call.c: In function ‘__static_call_update’:
kernel/static_call.c:153:18: error: unused variable ‘mod’ [-Werror=unused-variable]
153 | struct module *mod = site_mod->mod;
| ^~~
cc1: all warnings being treated as errors
make[1]: *** [scripts/Makefile.build:271: kernel/static_call.o] Error 1

This is simply because since recently, we no longer use 'mod' variable
elsewhere if MODULE is unset.

When using 'make tinyconfig' to generate the default kconfig, MODULE is
unset.

There are different ways to fix this warning. Here I tried to minimised
the number of modified lines and not add more #ifdef. We could also move
the declaration of the 'mod' variable inside the if-statement or
directly use site_mod->mod.

Fixes: 698bacefe993 ("static_call: Align static_call_is_init() patching condition")
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210326105023.2058860-1-matthieu.baerts@tessares.net

static_call: Fix static_call_update() sanity check

Sites that match init_section_contains() get marked as INIT. For
built-in code init_sections contains both __init and __exit text. OTOH
kernel_text_address() only explicitly includes __init text (and there
are no __exit text markers).

Match what jump_label already does and ignore the warning for INIT
sites. Also see the excellent changelog for commit: 8f35eaa5f2de
("jump_label: Don't warn on __exit jump entries")

Fixes: 9183c3f9ed710 ("static_call: Add inline static call infrastructure")
Reported-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Jarkko Sakkinen <jarkko@kernel.org>
Tested-by: Sumit Garg <sumit.garg@linaro.org>
Link: https://lkml.kernel.org/r/20210318113610.739542434@infradead.org

static_call: Align static_call_is_init() patching condition

The intent is to avoid writing init code after init (because the text
might have been freed). The code is needlessly different between
jump_label and static_call and not obviously correct.

The existing code relies on the fact that the module loader clears the
init layout, such that within_module_init() always fails, while
jump_label relies on the module state which is more obvious and
matches the kernel logic.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Jarkko Sakkinen <jarkko@kernel.org>
Tested-by: Sumit Garg <sumit.garg@linaro.org>
Link: https://lkml.kernel.org/r/20210318113610.636651340@infradead.org

static_call: Fix static_call_set_init()

It turns out that static_call_set_init() does not preserve the other
flags; IOW. it clears TAIL if it was set.

Fixes: 9183c3f9ed710 ("static_call: Add inline static call infrastructure")
Reported-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Jarkko Sakkinen <jarkko@kernel.org>
Tested-by: Sumit Garg <sumit.garg@linaro.org>
Link: https://lkml.kernel.org/r/20210318113610.519406371@infradead.org

static_call: Fix the module key fixup

Provided the target address of a R_X86_64_PC32 relocation is aligned,
the low two bits should be invariant between the relative and absolute
value.

Turns out the address is not aligned and things go sideways, ensure we
transfer the bits in the absolute form when fixing up the key address.

Fixes: 73f44fe19d35 ("static_call: Allow module use without exposing static_call_key")
Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Tested-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Link: https://lkml.kernel.org/r/20210225220351.GE4746@worktop.programming.kicks-ass.net

static_call: Allow module use without exposing static_call_key

When exporting static_call_key; with EXPORT_STATIC_CALL*(), the module
can use static_call_update() to change the function called. This is
not desirable in general.

Not exporting static_call_key however also disallows usage of
static_call(), since objtool needs the key to construct the
static_call_site.

Solve this by allowing objtool to create the static_call_site using
the trampoline address when it builds a module and cannot find the
static_call_key symbol. The module loader will then try and map the
trampole back to a key before it constructs the normal sites list.

Doing this requires a trampoline -> key associsation, so add another
magic section that keeps those.

Originally-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lkml.kernel.org/r/20210127231837.ifddpn7rhwdaepiu@treble

static_call/x86: Add __static_call_return0()

Provide a stub function that return 0 and wire up the static call site
patching to replace the CALL with a single 5 byte instruction that
clears %RAX, the return value register.

The function can be cast to any function pointer type that has a
single %RAX return (including pointers). Also provide a version that
returns an int for convenience. We are clearing the entire %RAX register
in any case, whether the return value is 32 or 64 bits, since %RAX is
always a scratch register anyway.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lkml.kernel.org/r/20210118141223.123667-2-frederic@kernel.org

static_call: Fix return type of static_call_init

Functions that are passed to early_initcall should be of type
initcall_t, which expects a return type of int. This is not currently an
error but a patch in the Clang LTO series could change that in the
future.

Fixes: 9183c3f9ed71 ("static_call: Add inline static call infrastructure")
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Link: https://lore.kernel.org/lkml/20200903203053.3411268-17-samitolvanen@google.com/

static_call: Allow early init

In order to use static_call() to wire up x86_pmu, we need to
initialize earlier, specifically before memory allocation works; copy
some of the tricks from jump_label to enable this.

Primarily we overload key->next to store a sites pointer when there
are no modules, this avoids having to use kmalloc() to initialize the
sites and allows us to run much earlier.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Link: https://lore.kernel.org/r/20200818135805.220737930@infradead.org

static_call: Handle tail-calls

GCC can turn our static_call(name)(args...) into a tail call, in which
case we get a JMP.d32 into the trampoline (which then does a further
tail-call).

Teach objtool to recognise and mark these in .static_call_sites and
adjust the code patching to deal with this.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20200818135805.101186767@infradead.org

static_call: Add simple self-test for static calls

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200818135804.922581202@infradead.org

static_call: Avoid kprobes on inline static_call()s

Similar to how we disallow kprobes on any other dynamic text
(ftrace/jump_label) also disallow kprobes on inline static_call()s.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200818135804.744920586@infradead.org

static_call: Add inline static call infrastructure

Add infrastructure for an arch-specific CONFIG_HAVE_STATIC_CALL_INLINE
option, which is a faster version of CONFIG_HAVE_STATIC_CALL. At
runtime, the static call sites are patched directly, rather than using
the out-of-line trampolines.

Compared to out-of-line static calls, the performance benefits are more
modest, but still measurable. Steven Rostedt did some tracepoint
measurements:

https://lkml.kernel.org/r/20181126155405.72b4f718@gandalf.local.home

This code is heavily inspired by the jump label code (aka "static
jumps"), as some of the concepts are very similar.

For more details, see the comments in include/linux/static_call.h.

[peterz: simplified interface; merged trampolines]

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20200818135804.684334440@infradead.org