Message-ID: <20170815113625.GA2738@pathway.suse.cz>
Date:   Tue, 15 Aug 2017 13:36:25 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Helge Deller <deller@....de>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org, linux-parisc@...r.kernel.org
Subject: Re: [PATCH] printk-formats.txt: Add examples for %pS and %pF

On Fri 2017-08-11 09:31:28, Helge Deller wrote:
> On 11.08.2017 02:15, Sergey Senozhatsky wrote:
> > On (08/10/17 19:35), Helge Deller wrote:
> >> Sometimes people seems unclear when to use the %pS or %pF printk format.
> >> Adding some examples may help to avoid such mistakes.
> >>
> >> See for example commit 51d96dc2e2dc ("random: fix warning message on ia64 and
> >> parisc") which fixed such a wrong format string.
> >>
> >> Signed-off-by: Helge Deller <deller@....de>
> >>
> >> diff --git a/Documentation/printk-formats.txt b/Documentation/printk-formats.txt
> >> index 65ea591..be8c05b 100644
> >> --- a/Documentation/printk-formats.txt
> >> +++ b/Documentation/printk-formats.txt
> >> @@ -73,6 +73,12 @@ actually function descriptors which must first be resolved. The ``F`` and
> >>  ``f`` specifiers perform this resolution and then provide the same
> >>  functionality as the ``S`` and ``s`` specifiers.
> >>  
> >> +Examples::
> >> +
> >> +	printk("Called from %pS.\n", __builtin_return_address(0));
> >> +	printk("Called from %pS.\n", (void *)regs->ip);
> >> +	printk("Called from %pF.\n", &gettimeofday);
> > 
> > there is this paragraph
> > 
> > : On ia64, ppc64 and parisc64 architectures function pointers are
> > : actually function descriptors which must first be resolved. The ``F`` and
> > : ``f`` specifiers perform this resolution and then provide the same
> > : functionality as the ``S`` and ``s`` specifiers.
> > 
> > which supposed to explain everything in details. the examples
> > don't make it any `clearer', IMHO.
> 
> Experts surely do know what function descriptors are.
> Nevertheless even those often get it wrong as can be seen in
> various commits.

It seems that these specifiers are used the wrong way on many
locations. They might be worth fixing but I cannot test it
easily.

Hmm, using %pF might actually cause a crash when used
on direct function address.


> The hope with this patch is to show widely-used examples
> and avoid additional commits afterwards to fix it up.

IMHO, one problem is that the meaning of ''F'' and ''f''
is hidden at the end of the section. Also the first line

  'For printing symbols and function pointers. The ``S`` and ``s`` '

kind of invites to use ``S`` and ``s`` even for function pointers.
I suggest to switch the order, slightly retranslate, add the
examples, see below.


> This patch was meant to be RFC.
> If you decide not to take it, I'm fine as well.
> 
> > *may be* on "ia64, ppc64 and parisc64" we can somehow check
> > that the pointer, which we pass as %pS, belongs to .text and
> > print some build-time warnings. well, if it's actually a
> > problem. dunno.

I think that it would need to be a runtime check because many/most
printed addresses are not statically defined.


> I think it's not needed. Those bugs will be seen and fixed.

I am not sure how many people are familiar with this problem.
I might help to avoid some headaches when debugging.

If we add the warning, it should be ratelimited to reduce messing
of the original message.

I do not have strong opinion about it.


Here is the updated patch with my proposed changes.
Feel free to update it:


>From ef983c65095cada994c1fe531e2b98e936c943bf Mon Sep 17 00:00:00 2001
From: Helge Deller <deller@....de>
Date: Tue, 15 Aug 2017 11:34:19 +0200
Subject: [PATCH] printk-formats.txt: Better describe the difference between
 %pS and %pF

Sometimes people seems unclear when to use the %pS or %pF printk format.
For example, see commit 51d96dc2e2dc ("random: fix warning message on ia64
and parisc") which fixed such a wrong format string.

The documentation should be more clear about the difference.

Signed-off-by: Helge Deller <deller@....de>
[pmladek@...e.com: Restructure the entire section]
Signed-off-by: Petr Mladek <pmladek@...e.com>
---
 Documentation/printk-formats.txt | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/Documentation/printk-formats.txt b/Documentation/printk-formats.txt
index 65ea5915178b..074670b98bac 100644
--- a/Documentation/printk-formats.txt
+++ b/Documentation/printk-formats.txt
@@ -58,20 +58,23 @@ Symbols/Function Pointers
 	%ps	versatile_init
 	%pB	prev_fn_of_versatile_init+0x88/0x88
 
-For printing symbols and function pointers. The ``S`` and ``s`` specifiers
-result in the symbol name with (``S``) or without (``s``) offsets. Where
-this is used on a kernel without KALLSYMS - the symbol address is
-printed instead.
+The ``F`` and ``f`` specifiers are for printing function pointers,
+for example, f->func, &gettimeofday. They have the same result as
+``S`` and ``s`` specifiers. But they do an extra conversion on
+ia64, ppc64 and parisc64 architectures where the function pointers
+are actually function descriptors.
+
+The ``S`` and ``s`` specifiers can be used for printing symbols
+from direct addresses, for example, __builtin_return_address(0),
+(void *)regs->ip. They result in the symbol name with (``S``) or
+without (``s``) offsets. If KALLSYMS are disabled then the symbol
+address is printed instead.
 
 The ``B`` specifier results in the symbol name with offsets and should be
 used when printing stack backtraces. The specifier takes into
 consideration the effect of compiler optimisations which may occur
 when tail-call``s are used and marked with the noreturn GCC attribute.
 
-On ia64, ppc64 and parisc64 architectures function pointers are
-actually function descriptors which must first be resolved. The ``F`` and
-``f`` specifiers perform this resolution and then provide the same
-functionality as the ``S`` and ``s`` specifiers.
 
 Kernel Pointers
 ===============
-- 
1.8.5.6

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