Le 22/06/2025 à 18:35, David Laight a écrit :
On Sun, 22 Jun 2025 11:52:39 +0200
Christophe Leroy <christophe.le...@csgroup.eu> wrote:

Allthough masked_user_access_begin() seems to only be used when reading
data from user at the moment, introduce masked_user_read_access_begin()
and masked_user_write_access_begin() in order to match
user_read_access_begin() and user_write_access_begin().

Have them default to masked_user_access_begin() when they are
not defined.

Signed-off-by: Christophe Leroy <christophe.le...@csgroup.eu>
---
  fs/select.c             | 2 +-
  include/linux/uaccess.h | 8 ++++++++
  kernel/futex/futex.h    | 4 ++--
  lib/strncpy_from_user.c | 2 +-
  lib/strnlen_user.c      | 2 +-
  5 files changed, 13 insertions(+), 5 deletions(-)

diff --git a/fs/select.c b/fs/select.c
index 9fb650d03d52..d8547bedf5eb 100644
--- a/fs/select.c
+++ b/fs/select.c
@@ -777,7 +777,7 @@ static inline int get_sigset_argpack(struct sigset_argpack 
*to,
        // the path is hot enough for overhead of copy_from_user() to matter
        if (from) {
                if (can_do_masked_user_access())
-                       from = masked_user_access_begin(from);
+                       from = masked_user_read_access_begin(from);
                else if (!user_read_access_begin(from, sizeof(*from)))
                        return -EFAULT;
                unsafe_get_user(to->p, &from->p, Efault);
diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
index 7c06f4795670..682a0cd2fe51 100644
--- a/include/linux/uaccess.h
+++ b/include/linux/uaccess.h
@@ -41,6 +41,14 @@

  #ifdef masked_user_access_begin
   #define can_do_masked_user_access() 1
  #else
   #define can_do_masked_user_access() 0
   #define masked_user_access_begin(src) NULL
   #define mask_user_address(src) (src)
  #endif
+#ifndef masked_user_write_access_begin
+#define masked_user_write_access_begin masked_user_access_begin
+#endif
+#ifndef masked_user_read_access_begin
+#define masked_user_read_access_begin masked_user_access_begin
+#endif

I think that needs merging with the bit above.
Perhaps generating something like:

#ifdef masked_user_access_begin
#define masked_user_read_access_begin masked_user_access_begin
#define masked_user_write_access_begin masked_user_access_begin
#endif

#ifdef masked_user_read_access_begin
   #define can_do_masked_user_access() 1
#else
   #define can_do_masked_user_access() 0
   #define masked_user_read_access_begin(src) NULL
   #define masked_user_write_access_begin(src) NULL
   #define mask_user_address(src) (src)
#endif

Otherwise you'll have to #define masked_user_access_begin even though
it is never used.

I'm not sure I understand what you mean.

masked_user_access_begin() is used, for instance in arch/x86/include/asm/futex.h so it will remain.

masked_user_access_begin() is the analogy of user_access_begin(), it starts a read-write user access and is worth it.


Two more patches could change x86-64 to define both and then remove
the 'then unused' first check - but that has to be for later.


Christophe

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