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Date: Fri, 10 Jul 2020 20:23:19 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, mhiramat@...nel.org,
bristot@...hat.com, jbaron@...mai.com,
torvalds@...ux-foundation.org, tglx@...utronix.de,
mingo@...nel.org, namit@...are.com, hpa@...or.com, luto@...nel.org,
ard.biesheuvel@...aro.org, jpoimboe@...hat.com,
pbonzini@...hat.com, mathieu.desnoyers@...icios.com,
linux@...musvillemoes.dk
Subject: Re: [PATCH v6 14/17] static_call: Handle tail-calls
On Fri, 10 Jul 2020 15:38:45 +0200
Peter Zijlstra <peterz@...radead.org> wrote:
> 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.
>
Hmm, were you able to trigger crashes before this patch?
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
> arch/x86/kernel/static_call.c | 21 ++++++++++++++++++---
> include/linux/static_call.h | 4 ++--
> include/linux/static_call_types.h | 7 +++++++
> kernel/static_call.c | 21 +++++++++++++--------
> tools/include/linux/static_call_types.h | 7 +++++++
> tools/objtool/check.c | 18 +++++++++++++-----
> 6 files changed, 60 insertions(+), 18 deletions(-)
>
> --- a/arch/x86/kernel/static_call.c
> +++ b/arch/x86/kernel/static_call.c
> @@ -41,15 +41,30 @@ static void __static_call_transform(void
> text_poke_bp(insn, code, size, NULL);
> }
>
> -void arch_static_call_transform(void *site, void *tramp, void *func)
> +static inline enum insn_type __sc_insn(bool null, bool tail)
> +{
> + /*
> + * Encode the following table without branches:
> + *
> + * tail null insn
> + * -----+-------+------
> + * 0 | 0 | CALL
> + * 0 | 1 | NOP
> + * 1 | 0 | JMP
> + * 1 | 1 | RET
> + */
> + return 2*tail + null;
> +}
> +
> +void arch_static_call_transform(void *site, void *tramp, void *func, bool tail)
> {
> mutex_lock(&text_mutex);
>
> if (tramp)
> - __static_call_transform(tramp, func ? JMP : RET, func);
> + __static_call_transform(tramp, __sc_insn(!func, true), func);
>
> if (IS_ENABLED(CONFIG_HAVE_STATIC_CALL_INLINE) && site)
> - __static_call_transform(site, func ? CALL : NOP, func);
> + __static_call_transform(site, __sc_insn(!func, tail), func);
>
> mutex_unlock(&text_mutex);
> }
> --- a/include/linux/static_call.h
> +++ b/include/linux/static_call.h
> @@ -103,7 +103,7 @@
> /*
> * Either @site or @tramp can be NULL.
> */
> -extern void arch_static_call_transform(void *site, void *tramp, void *func);
> +extern void arch_static_call_transform(void *site, void *tramp, void *func, bool tail);
>
> #define STATIC_CALL_TRAMP_ADDR(name) &STATIC_CALL_TRAMP(name)
>
> @@ -206,7 +206,7 @@ void __static_call_update(struct static_
> {
> cpus_read_lock();
> WRITE_ONCE(key->func, func);
> - arch_static_call_transform(NULL, tramp, func);
> + arch_static_call_transform(NULL, tramp, func, false);
> cpus_read_unlock();
> }
>
> --- a/include/linux/static_call_types.h
> +++ b/include/linux/static_call_types.h
> @@ -17,6 +17,13 @@
> #define STATIC_CALL_TRAMP_STR(name) __stringify(STATIC_CALL_TRAMP(name))
>
> /*
> + * Flags in the low bits of static_call_site::key.
> + */
> +#define STATIC_CALL_SITE_TAIL 1UL /* tail call */
> +#define STATIC_CALL_SITE_INIT 2UL /* init section */
> +#define STATIC_CALL_SITE_FLAGS 3UL
> +
> +/*
> * The static call site table needs to be created by external tooling (objtool
> * or a compiler plugin).
> */
> --- a/kernel/static_call.c
> +++ b/kernel/static_call.c
> @@ -15,8 +15,6 @@ extern struct static_call_site __start_s
>
> static bool static_call_initialized;
>
> -#define STATIC_CALL_INIT 1UL
> -
> /* mutex to protect key modules/sites */
> static DEFINE_MUTEX(static_call_mutex);
>
> @@ -39,18 +37,23 @@ static inline void *static_call_addr(str
> static inline struct static_call_key *static_call_key(const struct static_call_site *site)
> {
> return (struct static_call_key *)
> - (((long)site->key + (long)&site->key) & ~STATIC_CALL_INIT);
> + (((long)site->key + (long)&site->key) & ~STATIC_CALL_SITE_FLAGS);
> }
>
> /* These assume the key is word-aligned. */
> static inline bool static_call_is_init(struct static_call_site *site)
> {
> - return ((long)site->key + (long)&site->key) & STATIC_CALL_INIT;
> + return ((long)site->key + (long)&site->key) & STATIC_CALL_SITE_INIT;
> +}
> +
> +static inline bool static_call_is_tail(struct static_call_site *site)
> +{
> + return ((long)site->key + (long)&site->key) & STATIC_CALL_SITE_TAIL;
> }
>
> static inline void static_call_set_init(struct static_call_site *site)
> {
> - site->key = ((long)static_call_key(site) | STATIC_CALL_INIT) -
> + site->key = ((long)static_call_key(site) | STATIC_CALL_SITE_INIT) -
> (long)&site->key;
> }
>
> @@ -104,7 +107,7 @@ void __static_call_update(struct static_
>
> key->func = func;
>
> - arch_static_call_transform(NULL, tramp, func);
> + arch_static_call_transform(NULL, tramp, func, false);
>
> /*
> * If uninitialized, we'll not update the callsites, but they still
> @@ -154,7 +157,8 @@ void __static_call_update(struct static_
> continue;
> }
>
> - arch_static_call_transform(site_addr, NULL, func);
> + arch_static_call_transform(site_addr, NULL, func,
> + static_call_is_tail(site));
> }
> }
>
> @@ -198,7 +202,8 @@ static int __static_call_init(struct mod
> key->mods = site_mod;
> }
>
> - arch_static_call_transform(site_addr, NULL, key->func);
> + arch_static_call_transform(site_addr, NULL, key->func,
> + static_call_is_tail(site));
> }
>
> return 0;
> --- a/tools/include/linux/static_call_types.h
> +++ b/tools/include/linux/static_call_types.h
> @@ -17,6 +17,13 @@
> #define STATIC_CALL_TRAMP_STR(name) __stringify(STATIC_CALL_TRAMP(name))
>
> /*
> + * Flags in the low bits of static_call_site::key.
> + */
> +#define STATIC_CALL_SITE_TAIL 1UL /* tail call */
> +#define STATIC_CALL_SITE_INIT 2UL /* init section */
> +#define STATIC_CALL_SITE_FLAGS 3UL
> +
> +/*
> * The static call site table needs to be created by external tooling (objtool
> * or a compiler plugin).
> */
> --- a/tools/objtool/check.c
> +++ b/tools/objtool/check.c
> @@ -511,7 +511,7 @@ static int create_static_call_sections(s
> }
> memset(reloc, 0, sizeof(*reloc));
> reloc->sym = key_sym;
> - reloc->addend = 0;
> + reloc->addend = is_sibling_call(insn) ? STATIC_CALL_SITE_TAIL : 0;
> reloc->type = R_X86_64_PC32;
> reloc->offset = idx * sizeof(struct static_call_site) + 4;
> reloc->sec = reloc_sec;
> @@ -720,6 +720,10 @@ static int add_jump_destinations(struct
> } else {
> /* external sibling call */
> insn->call_dest = reloc->sym;
> + if (insn->call_dest->static_call_tramp) {
> + list_add_tail(&insn->static_call_node,
> + &file->static_call_list);
> + }
> continue;
> }
>
> @@ -771,6 +775,10 @@ static int add_jump_destinations(struct
>
> /* internal sibling call */
> insn->call_dest = insn->jump_dest->func;
> + if (insn->call_dest->static_call_tramp) {
> + list_add_tail(&insn->static_call_node,
> + &file->static_call_list);
> + }
> }
> }
> }
> @@ -1639,6 +1647,10 @@ static int decode_sections(struct objtoo
> if (ret)
> return ret;
>
> + ret = read_static_call_tramps(file);
> + if (ret)
> + return ret;
> +
> ret = add_jump_destinations(file);
> if (ret)
> return ret;
> @@ -1671,10 +1683,6 @@ static int decode_sections(struct objtoo
> if (ret)
> return ret;
>
> - ret = read_static_call_tramps(file);
> - if (ret)
> - return ret;
Hmm, what's the reason for moving this above? Should we have a comment
here if there's importance that read_static_call_trampoline() is done
earlier?
-- Steve
> -
> return 0;
> }
>
>
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