Re: [PATCH] riscv: Fix early ftrace nop patching
From: Alexandre Ghiti
Date: Mon Jun 24 2024 - 04:09:08 EST
On 20/06/2024 19:03, Alexandre Ghiti wrote:
On 19/06/2024 05:40, Andy Chiu wrote:
On Tue, Jun 18, 2024 at 9:40 PM Alexandre Ghiti
<alexghiti@xxxxxxxxxxxx> wrote:
Hi Andy,
On Tue, Jun 18, 2024 at 2:48 PM Andy Chiu <andy.chiu@xxxxxxxxxx> wrote:
On Tue, Jun 18, 2024 at 8:02 PM Alexandre Ghiti <alex@xxxxxxxx> wrote:
Hi Conor,
On 17/06/2024 15:23, Alexandre Ghiti wrote:
Hi Conor,
Sorry for the delay here.
On 13/06/2024 09:48, Conor Dooley wrote:
On Thu, May 23, 2024 at 01:51:34PM +0200, Alexandre Ghiti wrote:
Commit c97bf629963e ("riscv: Fix text patching when IPI are used")
converted ftrace_make_nop() to use patch_insn_write() which
does not
emit any icache flush relying entirely on
__ftrace_modify_code() to do
that.
But we missed that ftrace_make_nop() was called very early
directly
when
converting mcount calls into nops (actually on riscv it
converts 2B
nops
emitted by the compiler into 4B nops).
This caused crashes on multiple HW as reported by Conor and
Björn since
the booting core could have half-patched instructions in its
icache
which would trigger an illegal instruction trap: fix this by
emitting a
local flush icache when early patching nops.
Fixes: c97bf629963e ("riscv: Fix text patching when IPI are used")
Signed-off-by: Alexandre Ghiti <alexghiti@xxxxxxxxxxxx>
---
arch/riscv/include/asm/cacheflush.h | 6 ++++++
arch/riscv/kernel/ftrace.c | 3 +++
2 files changed, 9 insertions(+)
diff --git a/arch/riscv/include/asm/cacheflush.h
b/arch/riscv/include/asm/cacheflush.h
index dd8d07146116..ce79c558a4c8 100644
--- a/arch/riscv/include/asm/cacheflush.h
+++ b/arch/riscv/include/asm/cacheflush.h
@@ -13,6 +13,12 @@ static inline void local_flush_icache_all(void)
asm volatile ("fence.i" ::: "memory");
}
+static inline void local_flush_icache_range(unsigned long
start,
+ unsigned long end)
+{
+ local_flush_icache_all();
+}
+
#define PG_dcache_clean PG_arch_1
static inline void flush_dcache_folio(struct folio *folio)
diff --git a/arch/riscv/kernel/ftrace.c
b/arch/riscv/kernel/ftrace.c
index 4f4987a6d83d..32e7c401dfb4 100644
--- a/arch/riscv/kernel/ftrace.c
+++ b/arch/riscv/kernel/ftrace.c
@@ -120,6 +120,9 @@ int ftrace_init_nop(struct module *mod, struct
dyn_ftrace *rec)
out = ftrace_make_nop(mod, rec, MCOUNT_ADDR);
mutex_unlock(&text_mutex);
So, turns out that this patch is not sufficient. I've seen some
more
crashes, seemingly due to initialising nops on this mutex_unlock().
Palmer suggested moving the if (!mod) ... into the lock, which
fixed
the problem for me.
Ok, it makes sense, I completely missed that. I'll send a fix for
that
shortly so that it can be merged in rc5.
Thanks,
Alex
So I digged a bit more and I'm afraid that the only way to make sure
this issue does not happen elsewhere is to flush the icache right
after
the patching. We actually can't wait to batch the icache flush since
along the way, we may call a function that has just been patched (the
issue that you encountered here).
Trying to provide my thoughts, please let me know if I missed
anything. I think what Conor suggested is safe for init_nop, as it
would be called only when there is only one core (booting) and at the
loading stage of kernel modules. In the first case we just have to
make sure there is no patchable entry before the core executes
fence.i. The second case is unconditionally safe because there is no
read-side of the race.
It is a bit tricky for the generic (runtime) case of ftrace code
patching, but that is not because of the batch fence.i maintenance. As
long as there exists a patchable entry for the stopping thread, it is
possible for them to execute on a partially patched instruction. A
solution for this is again to prevent any patchable entry in the
stop_machine's stopping thread. Another solution is to apply the
atomic ftrace patching series which aims to get rid of the race.
Yeah but my worry is that we can't make sure that functions called
between the patching and the fence.i are not the ones that just get
patched. At least today, patch_insn_write() etc should be marked as
IIUC, the compiler does not generate a patchable entry for
patch_insn_write, and so do all functions in patch.c. The entire file
is not compiled with patchable entry because the flag is removed in
riscv's Makefile. Please let me know if I misunderstand it.
noinstr. But even then, we call core functions that could be patchable
(I went through all and it *seems* we are safe *for now*, but this is
very fragile).
Yes, functions in the "else" clause, atomic_read() etc, are safe for
now. However, it is impossible to fix as long as there exists a
patchable entry along the way. This is the problem that we cannot
patch 2 instructions with a concurrent read-side. On the other hand,
the path of ftrace_modify_all_code may experience the batch fence.i
maintenance issue, and can be fixed via a local fence.i. This is
because the path is executed by only one core. There does not exist a
concurrent write-side. As a result, we just need to make sure it
leaves each patch-site with a local fence.i to make sure code is
up-to-date.
Then what do you think we should do to fix Conor's issue right now:
should we mark the riscv specific functions as noinstr, cross fingers
that the core functions are not (and don't become) patchable and wait
for your atomic patching? Or should we flush as soon as possible as I
proposed above (which to me fixes the issue but hurts performance)?
Either way works for me. IMHO, the fix should include:
1. move fence.i before mutex_unlock in init_nop
2. do local fence.i in __ftrace_modify_call
3. make sure the else clause does not happen to have a patchable entry
OK I have just checked again and I agree with you on the whole fix :)
I may add some comments for 3 so that it is very clear.
And after another check, a few other callsites need to emit a sfence.i
like ftrace_make_call() and ftrace_make_nop(), which in the end, amounts
to adding a sfence.i right after the patching is done. So after a
private discussion with Andy, we both agreed that the initial solution I
posted was the safest. Admittedly, the impact on performance is unknown,
so any input on this side would be appreciated.
@Conor: I'm sending this as a proper patch if you can give it a try so
that we can merge that in rc6.
Thanks,
Alex
Thanks Andy!
Alex
Thanks,
Andy
Thanks,
Alex
I don't know how much it will impact the performance but I guess
it will.
Unless someone has a better idea (I added Andy and Puranjay in
cc), here
is the patch that implements this, can you give it a try? Thanks
diff --git a/arch/riscv/kernel/ftrace.c b/arch/riscv/kernel/ftrace.c
index 87cbd86576b2..4b95c574fd04 100644
--- a/arch/riscv/kernel/ftrace.c
+++ b/arch/riscv/kernel/ftrace.c
@@ -120,9 +120,6 @@ int ftrace_init_nop(struct module *mod, struct
dyn_ftrace *rec)
out = ftrace_make_nop(mod, rec, MCOUNT_ADDR);
mutex_unlock(&text_mutex);
- if (!mod)
- local_flush_icache_range(rec->ip, rec->ip +
MCOUNT_INSN_SIZE);
-
return out;
}
@@ -156,9 +153,9 @@ static int __ftrace_modify_code(void *data)
} else {
while (atomic_read(¶m->cpu_count) <=
num_online_cpus())
cpu_relax();
- }
- local_flush_icache_all();
+ local_flush_icache_all();
+ }
return 0;
}
diff --git a/arch/riscv/kernel/patch.c b/arch/riscv/kernel/patch.c
index 4007563fb607..ab03732d06c4 100644
--- a/arch/riscv/kernel/patch.c
+++ b/arch/riscv/kernel/patch.c
@@ -89,6 +89,14 @@ static int __patch_insn_set(void *addr, u8 c,
size_t len)
memset(waddr, c, len);
+ /*
+ * We could have just patched a function that is about to be
+ * called so make sure we don't execute partially patched
+ * instructions by flushing the icache as soon as possible.
+ */
+ local_flush_icache_range((unsigned long)waddr,
+ (unsigned long)waddr + len);
+
patch_unmap(FIX_TEXT_POKE0);
if (across_pages)
@@ -135,6 +143,14 @@ static int __patch_insn_write(void *addr, const
void *insn, size_t len)
ret = copy_to_kernel_nofault(waddr, insn, len);
+ /*
+ * We could have just patched a function that is about to be
+ * called so make sure we don't execute partially patched
+ * instructions by flushing the icache as soon as possible.
+ */
+ local_flush_icache_range((unsigned long)waddr,
+ (unsigned long)waddr + len);
+
patch_unmap(FIX_TEXT_POKE0);
if (across_pages)
@@ -189,9 +205,6 @@ int patch_text_set_nosync(void *addr, u8 c,
size_t len)
ret = patch_insn_set(tp, c, len);
- if (!ret)
- flush_icache_range((uintptr_t)tp, (uintptr_t)tp +
len);
-
return ret;
}
NOKPROBE_SYMBOL(patch_text_set_nosync);
@@ -224,9 +237,6 @@ int patch_text_nosync(void *addr, const void
*insns,
size_t len)
ret = patch_insn_write(tp, insns, len);
- if (!ret)
- flush_icache_range((uintptr_t) tp, (uintptr_t) tp
+ len);
-
return ret;
}
NOKPROBE_SYMBOL(patch_text_nosync);
@@ -253,9 +263,9 @@ static int patch_text_cb(void *data)
} else {
while (atomic_read(&patch->cpu_count) <=
num_online_cpus())
cpu_relax();
- }
- local_flush_icache_all();
+ local_flush_icache_all();
+ }
return ret;
}
+ if (!mod)
+ local_flush_icache_range(rec->ip, rec->ip +
MCOUNT_INSN_SIZE);
+
return out;
}
--
2.39.2
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Cheers,
Andy
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