| Summary: | fprintf() function is multithread-unsafe | ||
|---|---|---|---|
| Product: | glibc | Reporter: | Peng Haitao <penght> |
| Component: | stdio | Assignee: | Not yet assigned to anyone <unassigned> |
| Status: | NEW --- | ||
| Severity: | normal | CC: | aoliva, carlos, davem, drepper.fsp, fweimer, ondra |
| Priority: | P2 | Flags: | fweimer:
security-
|
| Version: | unspecified | ||
| Target Milestone: | --- | ||
| Host: | Target: | ||
| Build: | Last reconfirmed: | ||
| Attachments: | The Patch can make fprintf() function to multithread-safe | ||
This change should be rearranged such that we only take the lock if the feature in question is used, rather than unconditionally. Otherwise non-users of the feature take a performance hit, and that's undesirable. Peng, I agree with David, these changes need to be considered carefully because fprintf() is a function that everyone uses. Simply adding a lock here is going to impact performance for everyone. I suggest you follow Alex's posted patches and the discussion threads here: http://sourceware.org/ml/libc-alpha/2013-03/msg00581.html http://sourceware.org/ml/libc-alpha/2013-04/msg00023.html http://sourceware.org/ml/libc-alpha/2013-05/msg00120.html The first step is going to be to document the exact contract we want to have with our users, including possibly declaring that register_printf_specifier() is MT-unsafe, but that fprintf() *IS* MT-safe with exceptions (don't call register_printf_specifier). I encourage you to review Alex's work and provide feedback and help submit bugs where appropriate. |
Created attachment 6460 [details] The Patch can make fprintf() function to multithread-safe fprintf() uses static variables __printf_function_table and __printf_va_arg_table indirectly, which are not protected. It is not safe in multithread circumstance. When call fprintf() and register_printf_specifier() simultaneously in multithread circumstance, the following case will cause unsafe. The case has two threads: A and B. a. thread B call register_printf_specifier('W', print_widget, print_widget_arginfo) b. thread A call fprintf (stdout, "|%W|\n", &mywidget), when judge __printf_function_table is not NULL, but not output &mywidget c. thread B call register_printf_specifier('W', NULL, NULL) d. thread A output &mywidget when __printf_function_table is NULL e. programme will Segmentation fault