[ Upstream commit e66cfc29e0d06fec34c06bb40d4d281f595677b1 ]

Fix the following dead_code warning on some configurations in an
atomic development branch:

warning: constant `WARN_ON_FLAGS` is never used
   --> linux/rust/kernel/bug.rs:126:19
    |
126 |               const WARN_ON_FLAGS: u32 = $crate::bug::bugflag_taint($crate::bindings::TAINT_WARN);
    |                     ^^^^^^^^^^^^^
    |
   ::: linux/rust/kernel/sync/srcu.rs:106:12
    |
106 |           if crate::warn_on!(
    |  ____________-
107 | |             // SAFETY: By the type invariants, `self` contains a valid and pinned `struct srcu_struct`
108 | |             // and `srcu_readers_active()` only checks the active reader count.
109 | |             unsafe { bindings::srcu_readers_active(ptr) }
110 | |         ) {
    | |_________- in this macro invocation
    |
    = note: `#[warn(dead_code)]` (part of `#[warn(unused)]`) on by default
    = note: this warning originates in the macro `crate::warn_on` (in Nightly builds, run with -Z macro-backtrace for more info)

The warn_on! macro always defines a WARN_ON_FLAGS constant and hands it
to warn_flags!. On configurations where warn_flags! does not reference
its flags argument (the LOONGARCH/ARM variant, which only calls
WARN_ON(), and the !CONFIG_BUG no-op variant), the constant is left
unused and triggers a dead_code warning.

warn_flags! is the macro that accepts (and here discards) the flags
argument, so make it responsible for the argument it drops.

Also rename `_COND_STR` to `COND_STR` and consume `$file` for consistency.

Fixes: dff64b072708 ("rust: Add warn_on macro")
Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260801024841.786664-1-tomo@flapping.org
[ Added newlines. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
[ adapted unreachable flag consumption to the older one-argument warn_flags! interface. ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

commit 5d9668f3930609ead91f39b059ef4fe53db04942 upstream.

Running `make rusttest` with `ARCH=` set to an architecture other than
the host's fails, e.g. `ARCH=arm64` on an x86_64 host:

error: invalid instruction mnemonic 'brk'
  --> rust/kernel/bug.rs:63:17
   |
63 | /                 concat!(
64 | |                     "/* {size} */",
65 | |                     include!(concat!(env!("OBJTREE"), "/rust/kernel/generated_arch_warn_asm.rs")),
66 | |                     include!(concat!(env!("OBJTREE"), "/rust/kernel/generated_arch_reachable_asm.rs")));
   | |_______________________________________________________________________________________________________^
   |
note: instantiated into assembly here
  --> <inline asm>:1:115
   |
 1 |     /* 8 */.pushsection __bug_table,"aw"; .align 2; 14470: .long 14471f - .;.short 2305;.align 2; .popsection; 14471:brk 0x800
   |                                                                                                                      ^^^

The reason is that `rusttest` builds the `kernel` crate as a host
library: it passes the `CONFIG_*` cfgs of the configured architecture,
but not `--target`, so code generation happens for the
host. `warn_flags!` then selects the arch-specific inline asm arm
based on `CONFIG_*`, and the host assembler rejects it.

This does not happen with the current `master` because `warn_on!` has
no user inside the `kernel` crate itself yet, but it will as soon as
one is added.

Reported-by: Miguel Ojeda <ojeda@kernel.org>
Closes: https://lore.kernel.org/all/CANiq72n4=fz=JNKY0Jdm8BnLa=RmHB2B7s0bO47YTJ7hygqBZg@mail.gmail.com/
Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
Cc: stable@vger.kernel.org
Fixes: dff64b072708 ("rust: Add warn_on macro")
Link: https://patch.msgid.link/20260808022608.1125174-1-tomo@flapping.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Add warn_on macro, uses the BUG/WARN feature (lib/bug.c) via assembly
for x86_64/arm64/riscv.

The current Rust code simply wraps BUG() macro but doesn't provide the
proper debug information. The BUG/WARN feature can only be used from
assembly.

This uses the assembly code exported by the C side via ARCH_WARN_ASM
macro. To avoid duplicating the assembly code, this approach follows
the same strategy as the static branch code: it generates the assembly
code for Rust using the C preprocessor at compile time.

Similarly, ARCH_WARN_REACHABLE is also used at compile time to
generate the assembly code; objtool's reachable annotation code. It's
used for only architectures that use objtool.

For now, Loongarch and arm just use a wrapper for WARN macro.

UML doesn't use the assembly BUG/WARN feature; just wrapping generic
BUG/WARN functions implemented in C works.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250502094537.231725-5-fujita.tomonori@gmail.com
[ Avoid evaluating the condition twice (a good idea in general,
  but it also matches the C side). Simplify with `as_char_ptr()`
  to avoid a cast. Cast to `ffi` integer types for
  `warn_slowpath_fmt`. Avoid cast for `null()`. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>