Messages in this thread | | | Date | Thu, 28 May 2020 12:59:43 +0200 | From | Petr Mladek <> | Subject | Re: [PATCH] twist: allow converting pr_devel()/pr_debug() into printk(KERN_DEBUG) |
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On Thu 2020-05-28 08:33:37, Tetsuo Handa wrote: > On 2020/05/28 0:55, Petr Mladek wrote: > >>> Well, it would be possible to call vsprintf() with NULL buffer. It is > >>> normally used to calculate the length of the message before it is > >>> printed. But it also does all the accesses without printing anything. > >> > >> OK. I think that redirecting pr_debug() to vsnprintf(NULL, 0) will be > >> better than modifying dynamic_debug path, for > > > > It might get more complicated if you would actually want to see > > pr_debug() messages for a selected module in the fuzzer. > > I don't expect that automated testing can afford selectively enabling > DYNAMIC_DEBUG_BRANCH(id) conditions. But we could evaluate all arguments > by calling snprintf(NULL, 0) if the condition to call printk() is false. > > > vsprintf(NULL, ) can be called for pr_debug() messages in > > vprintk_store(). It will be again only a single place for > > all printk() wrappers. > > I couldn't catch what you mean. The problem of pr_debug() is that > vprintk_store() might not be called because of > > #define no_printk(fmt, ...) \ > ({ \ > if (0) \ > printk(fmt, ##__VA_ARGS__); \ > 0; \ > }) > > #define pr_debug(fmt, ...) \ > no_printk(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__) > > or > > #define __dynamic_func_call(id, fmt, func, ...) do { \ > DEFINE_DYNAMIC_DEBUG_METADATA(id, fmt); \ > if (DYNAMIC_DEBUG_BRANCH(id)) \ > func(&id, ##__VA_ARGS__); \ > } while (0) > > #define _dynamic_func_call(fmt, func, ...) \ > __dynamic_func_call(__UNIQUE_ID(ddebug), fmt, func, ##__VA_ARGS__) > > #define dynamic_pr_debug(fmt, ...) \ > _dynamic_func_call(fmt, __dynamic_pr_debug, \ > pr_fmt(fmt), ##__VA_ARGS__) > > #define pr_debug(fmt, ...) \ > dynamic_pr_debug(fmt, ##__VA_ARGS__)
That is exactly the problem. Your current patch [1] adds checks for the CONFIG_TWIST into 15 different locations.
This is perfectly fine for testing in linux-next whether this twist is worth the effort. But I do not like this as a long term solution.
If the testing shows that it was really helpful and you would want to get this into Linus' tree. Then I would like to do the twist at different level:
1. Add twist into ddebug_add_module() and enable all newly added entries by default. For example, by calling ddebug_exec_query("*:+p", const char *modname) or what is the syntax.
This will cause that any pr_devel() variant will always get called.
2. Add twist into vprintk_store(). In the current, implementation it would do:
#if TWIST return text_len; #endif
return log_output(facility, level, lflags, dict, dictlen, text, text_len);
Something similar would need to be done also in printk_safe(). Hot you could ignore this because it would be used only in very few scenarios.
In the lock_less variant, we would need to format the message into small buffer on stack to detect the log level from the first few bytes.
The approach will cause that pr_devel() message will never get really printed when this TWIST is enabled. But you mention that automatic testing would not do so anyway.
[1] https://lore.kernel.org/r/20200528065603.3596-1-penguin-kernel@I-love.SAKURA.ne.jp
Best Regards, Petr
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