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Message-ID: <CAEf4BzZx5iCAwS0iZv_0S1cADw840S8Ra=_PHPZhnW9cOYhFOQ@mail.gmail.com>
Date: Mon, 30 Sep 2024 13:58:01 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>, Andrii Nakryiko <andrii@...nel.org>,
Jiri Olsa <jolsa@...nel.org>, X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
alyssa.milburn@...el.com, scott.d.constable@...el.com,
Joao Moreira <joao@...rdrivepizza.com>, Andrew Cooper <andrew.cooper3@...rix.com>,
Josh Poimboeuf <jpoimboe@...nel.org>, "Jose E. Marchesi" <jose.marchesi@...cle.com>,
"H.J. Lu" <hjl.tools@...il.com>, Nick Desaulniers <ndesaulniers@...gle.com>,
Sami Tolvanen <samitolvanen@...gle.com>, Nathan Chancellor <nathan@...nel.org>, ojeda@...nel.org,
Kees Cook <kees@...nel.org>
Subject: Re: [PATCH 07/14] x86/ibt: Clean up is_endbr()
On Mon, Sep 30, 2024 at 2:33 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Mon, Sep 30, 2024 at 10:30:26AM +0200, Peter Zijlstra wrote:
> > On Sun, Sep 29, 2024 at 10:32:38AM -0700, Alexei Starovoitov wrote:
> > > On Fri, Sep 27, 2024 at 12:50 PM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > > > --- a/kernel/trace/bpf_trace.c
> > > > +++ b/kernel/trace/bpf_trace.c
> > > > @@ -1027,19 +1027,9 @@ static const struct bpf_func_proto bpf_g
> > > > #ifdef CONFIG_X86_KERNEL_IBT
> > > > static unsigned long get_entry_ip(unsigned long fentry_ip)
> > > > {
> > > > - u32 instr;
> > > > + if (is_endbr((void *)(fentry_ip - ENDBR_INSN_SIZE)))
> > > > + return fentry_ip - ENDBR_INSN_SIZE;
> > > >
> > > > - /* We want to be extra safe in case entry ip is on the page edge,
> > > > - * but otherwise we need to avoid get_kernel_nofault()'s overhead.
> > > > - */
> > > > - if ((fentry_ip & ~PAGE_MASK) < ENDBR_INSN_SIZE) {
> > > > - if (get_kernel_nofault(instr, (u32 *)(fentry_ip - ENDBR_INSN_SIZE)))
> > > > - return fentry_ip;
> > > > - } else {
> > > > - instr = *(u32 *)(fentry_ip - ENDBR_INSN_SIZE);
> > > > - }
> > > > - if (is_endbr(instr))
> > > > - fentry_ip -= ENDBR_INSN_SIZE;
> > > > return fentry_ip;
> > >
> > > Pls don't.
> > >
> > > This re-introduces the overhead that we want to avoid.
> > >
> > > Just call __is_endbr() here and keep the optimization.
> >
> > Well, I could do that ofcourse, but as I wrote elsewhere, the right
> > thing to do is to optimize get_kernel_nofault(), its current
> > implementation is needlessly expensive. All we really need is a load
> > with an exception entry, the STAC/CLAC and speculation nonsense should
> > not be needed.
>
> Looking at things, something like the below actually generates sane
> code:
>
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index a582cd25ca87..84f65ee9736c 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -1029,17 +1029,10 @@ static unsigned long get_entry_ip(unsigned long fentry_ip)
> {
> u32 instr;
>
> - /* We want to be extra safe in case entry ip is on the page edge,
> - * but otherwise we need to avoid get_kernel_nofault()'s overhead.
> - */
> - if ((fentry_ip & ~PAGE_MASK) < ENDBR_INSN_SIZE) {
> - if (get_kernel_nofault(instr, (u32 *)(fentry_ip - ENDBR_INSN_SIZE)))
> - return fentry_ip;
> - } else {
> - instr = *(u32 *)(fentry_ip - ENDBR_INSN_SIZE);
> - }
> + __get_kernel_nofault(&instr, (u32 *)(fentry_ip - ENDBR_INSN_SIZE), u32, Efault);
> if (is_endbr(instr))
> fentry_ip -= ENDBR_INSN_SIZE;
> +Efault:
> return fentry_ip;
> }
> #else
>
>
> Which then leads to me rewriting the proposed patch as...
>
> ---
> Subject: x86/ibt: Clean up is_endbr()
> From: Peter Zijlstra <peterz@...radead.org>
>
> Pretty much every caller of is_endbr() actually wants to test something at an
> address and ends up doing get_kernel_nofault(). Fold the lot into a more
> convenient helper.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
> arch/x86/events/core.c | 2 +-
> arch/x86/include/asm/ibt.h | 5 +++--
> arch/x86/kernel/alternative.c | 20 ++++++++++++++------
> arch/x86/kernel/kprobes/core.c | 11 +----------
> arch/x86/net/bpf_jit_comp.c | 4 ++--
> kernel/trace/bpf_trace.c | 14 ++------------
> 6 files changed, 23 insertions(+), 33 deletions(-)
>
> --- a/arch/x86/events/core.c
> +++ b/arch/x86/events/core.c
> @@ -2845,7 +2845,7 @@ static bool is_uprobe_at_func_entry(stru
> return true;
>
> /* endbr64 (64-bit only) */
> - if (user_64bit_mode(regs) && is_endbr(*(u32 *)auprobe->insn))
> + if (user_64bit_mode(regs) && is_endbr((u32 *)auprobe->insn))
> return true;
>
> return false;
> --- a/arch/x86/include/asm/ibt.h
> +++ b/arch/x86/include/asm/ibt.h
> @@ -65,7 +65,7 @@ static __always_inline __attribute_const
> return 0x001f0f66; /* osp nopl (%rax) */
> }
>
> -static inline bool is_endbr(u32 val)
> +static inline bool __is_endbr(u32 val)
> {
> if (val == gen_endbr_poison())
> return true;
> @@ -74,6 +74,7 @@ static inline bool is_endbr(u32 val)
> return val == gen_endbr();
> }
>
> +extern __noendbr bool is_endbr(u32 *val);
> extern __noendbr u64 ibt_save(bool disable);
> extern __noendbr void ibt_restore(u64 save);
>
> @@ -102,7 +103,7 @@ extern bool __do_kernel_cp_fault(struct
>
> #define __noendbr
>
> -static inline bool is_endbr(u32 val) { return false; }
> +static inline bool is_endbr(u32 *val) { return false; }
>
> static inline u64 ibt_save(bool disable) { return 0; }
> static inline void ibt_restore(u64 save) { }
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -852,16 +852,24 @@ void __init_or_module noinline apply_ret
>
> #ifdef CONFIG_X86_KERNEL_IBT
>
> +__noendbr bool is_endbr(u32 *val)
> +{
> + u32 endbr;
> +
> + __get_kernel_nofault(&endbr, val, u32, Efault);
> + return __is_endbr(endbr);
> +
> +Efault:
> + return false;
> +}
> +
> static void poison_cfi(void *addr);
>
> static void __init_or_module poison_endbr(void *addr, bool warn)
> {
> - u32 endbr, poison = gen_endbr_poison();
> -
> - if (WARN_ON_ONCE(get_kernel_nofault(endbr, addr)))
> - return;
> + u32 poison = gen_endbr_poison();
>
> - if (!is_endbr(endbr)) {
> + if (!is_endbr(addr)) {
> WARN_ON_ONCE(warn);
> return;
> }
> @@ -988,7 +996,7 @@ static u32 cfi_seed __ro_after_init;
> static u32 cfi_rehash(u32 hash)
> {
> hash ^= cfi_seed;
> - while (unlikely(is_endbr(hash) || is_endbr(-hash))) {
> + while (unlikely(__is_endbr(hash) || __is_endbr(-hash))) {
> bool lsb = hash & 1;
> hash >>= 1;
> if (lsb)
> --- a/arch/x86/kernel/kprobes/core.c
> +++ b/arch/x86/kernel/kprobes/core.c
> @@ -373,16 +373,7 @@ static bool can_probe(unsigned long padd
> kprobe_opcode_t *arch_adjust_kprobe_addr(unsigned long addr, unsigned long offset,
> bool *on_func_entry)
> {
> - u32 insn;
> -
> - /*
> - * Since 'addr' is not guaranteed to be safe to access, use
> - * copy_from_kernel_nofault() to read the instruction:
> - */
> - if (copy_from_kernel_nofault(&insn, (void *)addr, sizeof(u32)))
> - return NULL;
> -
> - if (is_endbr(insn)) {
> + if (is_endbr((u32 *)addr)) {
> *on_func_entry = !offset || offset == 4;
> if (*on_func_entry)
> offset = 4;
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -625,7 +625,7 @@ int bpf_arch_text_poke(void *ip, enum bp
> * See emit_prologue(), for IBT builds the trampoline hook is preceded
> * with an ENDBR instruction.
> */
> - if (is_endbr(*(u32 *)ip))
> + if (is_endbr(ip))
> ip += ENDBR_INSN_SIZE;
>
> return __bpf_arch_text_poke(ip, t, old_addr, new_addr);
> @@ -2971,7 +2971,7 @@ static int __arch_prepare_bpf_trampoline
> /* skip patched call instruction and point orig_call to actual
> * body of the kernel function.
> */
> - if (is_endbr(*(u32 *)orig_call))
> + if (is_endbr(orig_call))
> orig_call += ENDBR_INSN_SIZE;
> orig_call += X86_PATCH_SIZE;
> }
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -1027,19 +1027,9 @@ static const struct bpf_func_proto bpf_g
> #ifdef CONFIG_X86_KERNEL_IBT
> static unsigned long get_entry_ip(unsigned long fentry_ip)
> {
> - u32 instr;
> + if (is_endbr((void *)(fentry_ip - ENDBR_INSN_SIZE)))
> + return fentry_ip - ENDBR_INSN_SIZE;
>
> - /* We want to be extra safe in case entry ip is on the page edge,
> - * but otherwise we need to avoid get_kernel_nofault()'s overhead.
> - */
> - if ((fentry_ip & ~PAGE_MASK) < ENDBR_INSN_SIZE) {
> - if (get_kernel_nofault(instr, (u32 *)(fentry_ip - ENDBR_INSN_SIZE)))
> - return fentry_ip;
> - } else {
> - instr = *(u32 *)(fentry_ip - ENDBR_INSN_SIZE);
> - }
> - if (is_endbr(instr))
> - fentry_ip -= ENDBR_INSN_SIZE;
> return fentry_ip;
> }
> #else
LGTM.
Acked-by: Andrii Nakryiko <andrii@...nel.org>
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