On 19/02/2020 12.20, Ilya Dryomov wrote:

quoted

On Wed, Feb 19, 2020 at 9:21 AM Rasmus Villemoes
[off-list ref] wrote:

quoted

Extend %pe to pretty-print NULL in addition to ERR_PTRs,
i.e. everything IS_ERR_OR_NULL().

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
---
Something like this? The actual code change is +2,-1 with another +1
for a test case.

 Documentation/core-api/printk-formats.rst | 9 +++++----
 lib/errname.c                             | 4 ++++
 lib/test_printf.c                         | 1 +
 lib/vsprintf.c                            | 4 ++--
 4 files changed, 12 insertions(+), 6 deletions(-)

diff --git a/Documentation/core-api/printk-formats.rst b/Documentation/core-api/printk-formats.rst
index 8ebe46b1af39..964b55291445 100644
--- a/Documentation/core-api/printk-formats.rst
+++ b/Documentation/core-api/printk-formats.rst

@@ -86,10 +86,11 @@ Error Pointers

        %pe     -ENOSPC

-For printing error pointers (i.e. a pointer for which IS_ERR() is true)
-as a symbolic error name. Error values for which no symbolic name is
-known are printed in decimal, while a non-ERR_PTR passed as the
-argument to %pe gets treated as ordinary %p.
+For printing error pointers (i.e. a pointer for which IS_ERR() is
+true) as a symbolic error name. Error values for which no symbolic
+name is known are printed in decimal. A NULL pointer is printed as
+NULL. All other pointer values (i.e. anything !IS_ERR_OR_NULL()) get
+treated as ordinary %p.

 Symbols/Function Pointers
 -------------------------

diff --git a/lib/errname.c b/lib/errname.c
index 0c4d3e66170e..7757bc00f564 100644
--- a/lib/errname.c
+++ b/lib/errname.c

@@ -11,9 +11,13 @@
  * allocated errnos (with EHWPOISON = 257 on parisc, and EDQUOT = 1133
  * on mips), so this wastes a bit of space on those - though we
  * special case the EDQUOT case.
+ *
+ * For the benefit of %pe being able to print any ERR_OR_NULL pointer
+ * symbolically, 0 is also treated specially.
  */
 #define E(err) [err + BUILD_BUG_ON_ZERO(err <= 0 || err > 300)] = "-" #err
 static const char *names_0[] = {
+       [0] = "NULL",
        E(E2BIG),
        E(EACCES),
        E(EADDRINUSE),

diff --git a/lib/test_printf.c b/lib/test_printf.c
index 2d9f520d2f27..3a37d0e9e735 100644
--- a/lib/test_printf.c
+++ b/lib/test_printf.c

@@ -641,6 +641,7 @@ errptr(void)
        test("[-EIO    ]", "[%-8pe]", ERR_PTR(-EIO));
        test("[    -EIO]", "[%8pe]", ERR_PTR(-EIO));
        test("-EPROBE_DEFER", "%pe", ERR_PTR(-EPROBE_DEFER));
+       test("[NULL]", "[%pe]", NULL);
 #endif
 }

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 7c488a1ce318..b7118d78eb20 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c

@@ -2247,8 +2247,8 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,
        case 'x':
                return pointer_string(buf, end, ptr, spec);
        case 'e':
-               /* %pe with a non-ERR_PTR gets treated as plain %p */
-               if (!IS_ERR(ptr))
+               /* %pe with a non-ERR_OR_NULL ptr gets treated as plain %p */
+               if (!IS_ERR_OR_NULL(ptr))
                        break;

FWIW I was about to post a patch that just special cases NULL here.

I think changing errname() to return "NULL" for 0 is overkill.
People will sooner or later discover that function and start using it
in contexts that don't have anything to do with pointers.  Returning
_some_ string for 0 (instead of NULL) makes it very close to standard
strerror(), and "NULL" for 0 (i.e. success) seems rather odd.

I see what you mean, but I don't share your assumption that errname()
will ever grow callers other than the one in vsprintf.c. But I don't
have any strong opinion either way. Perhaps this on top of my patch

--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c

@@ -619,7 +619,7 @@ static char *err_ptr(char *buf, char *end, void *ptr,
                     struct printf_spec spec)
 {
        int err = PTR_ERR(ptr);
-       const char *sym = errname(err);
+       const char *sym = err ? errname(err) : "NULL";