Re: [PATCH -fixes] riscv: patch: Flush the icache right after patching to avoid illegal insns

[Palmer Dabbelt]

On Mon, 24 Jun 2024 01:21:41 PDT (-0700), alexgh...@rivosinc.com wrote:
We cannot delay the icache flush after patching some functions as we may
have patched a function that will get called before the icache flush.
The only way to completely avoid such scenario is by flushing the icache
as soon as we patch a function. This will probably be costly as we don't
batch the icache maintenance anymore.

Ya, it's going to be pretty miserable for performance.  We'd talked 
about using objtool for the static rewriting a few weeks ago in the 
patchwork meeting, but with the dynamic rewriting suffering from similar 
issues it seems best to just pick this one up.   We can always sort out 
the performance isuses later, at least this is correct.

Fixes: 6ca445d8af0e ("riscv: Fix early ftrace nop patching")
Reported-by: Conor Dooley <conor.doo...@microchip.com>
Closes: 
https://lore.kernel.org/linux-riscv/20240613-lubricant-breath-061192a9489a@wendy/
Signed-off-by: Alexandre Ghiti <alexgh...@rivosinc.com>
---
 arch/riscv/kernel/ftrace.c |  7 ++-----
 arch/riscv/kernel/patch.c  | 26 ++++++++++++++++++--------
 2 files changed, 20 insertions(+), 13 deletions(-)

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(&param->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;
 }