Re: [PATCH v3 25/34] s390/diag: Unpoison diag224() output buffer

[Heiko Carstens]

On Thu, Dec 14, 2023 at 12:24:45AM +0100, Ilya Leoshkevich wrote:
> Diagnose 224 stores 4k bytes, which cannot be deduced from the inline
> assembly constraints. This leads to KMSAN false positives.
> 
> Unpoison the output buffer manually with kmsan_unpoison_memory().
> 
> Signed-off-by: Ilya Leoshkevich <i...@linux.ibm.com>
> ---
>  arch/s390/kernel/diag.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/arch/s390/kernel/diag.c b/arch/s390/kernel/diag.c
> index 92fdc35f028c..fb83a21014d0 100644
> --- a/arch/s390/kernel/diag.c
> +++ b/arch/s390/kernel/diag.c
> @@ -9,6 +9,7 @@
>  #include <linux/export.h>
>  #include <linux/init.h>
>  #include <linux/cpu.h>
> +#include <linux/kmsan-checks.h>
>  #include <linux/seq_file.h>
>  #include <linux/debugfs.h>
>  #include <linux/vmalloc.h>
> @@ -255,6 +256,7 @@ int diag224(void *ptr)
>               "1:\n"
>               EX_TABLE(0b,1b)
>               : "+d" (rc) :"d" (0), "d" (addr) : "memory");
> +     kmsan_unpoison_memory(ptr, PAGE_SIZE);

Wouldn't it be better to adjust the inline assembly instead?
Something like this:

diff --git a/arch/s390/kernel/diag.c b/arch/s390/kernel/diag.c
index 92fdc35f028c..b1b0acda50c6 100644
--- a/arch/s390/kernel/diag.c
+++ b/arch/s390/kernel/diag.c
@@ -247,14 +247,18 @@ int diag224(void *ptr)
 {
        unsigned long addr = __pa(ptr);
        int rc = -EOPNOTSUPP;
+       struct _d {
+               char _d[4096];
+       };
 
        diag_stat_inc(DIAG_STAT_X224);
-       asm volatile(
-               "       diag    %1,%2,0x224\n"
-               "0:     lhi     %0,0x0\n"
+       asm volatile("\n"
+               "       diag    %[type],%[addr],0x224\n"
+               "0:     lhi     %[rc],0\n"
                "1:\n"
                EX_TABLE(0b,1b)
-               : "+d" (rc) :"d" (0), "d" (addr) : "memory");
+               : [rc] "+d" (rc), "=m" (*(struct _d *)ptr)
+               : [type] "d" (0), [addr] "d" (addr));
        return rc;
 }
 EXPORT_SYMBOL(diag224);