diff options
| -rw-r--r-- | Documentation/nommu-mmap.txt | 26 | ||||
| -rw-r--r-- | fs/binfmt_elf_fdpic.c | 3 | ||||
| -rw-r--r-- | include/asm-generic/mman-common.h | 5 | ||||
| -rw-r--r-- | init/Kconfig | 22 | ||||
| -rw-r--r-- | mm/nommu.c | 8 |
5 files changed, 60 insertions, 4 deletions
diff --git a/Documentation/nommu-mmap.txt b/Documentation/nommu-mmap.txt index b565e82..8e1ddec 100644 --- a/Documentation/nommu-mmap.txt +++ b/Documentation/nommu-mmap.txt @@ -119,6 +119,32 @@ FURTHER NOTES ON NO-MMU MMAP granule but will only discard the excess if appropriately configured as this has an effect on fragmentation. + (*) The memory allocated by a request for an anonymous mapping will normally + be cleared by the kernel before being returned in accordance with the + Linux man pages (ver 2.22 or later). + + In the MMU case this can be achieved with reasonable performance as + regions are backed by virtual pages, with the contents only being mapped + to cleared physical pages when a write happens on that specific page + (prior to which, the pages are effectively mapped to the global zero page + from which reads can take place). This spreads out the time it takes to + initialize the contents of a page - depending on the write-usage of the + mapping. + + In the no-MMU case, however, anonymous mappings are backed by physical + pages, and the entire map is cleared at allocation time. This can cause + significant delays during a userspace malloc() as the C library does an + anonymous mapping and the kernel then does a memset for the entire map. + + However, for memory that isn't required to be precleared - such as that + returned by malloc() - mmap() can take a MAP_UNINITIALIZED flag to + indicate to the kernel that it shouldn't bother clearing the memory before + returning it. Note that CONFIG_MMAP_ALLOW_UNINITIALIZED must be enabled + to permit this, otherwise the flag will be ignored. + + uClibc uses this to speed up malloc(), and the ELF-FDPIC binfmt uses this + to allocate the brk and stack region. + (*) A list of all the private copy and anonymous mappings on the system is visible through /proc/maps in no-MMU mode. diff --git a/fs/binfmt_elf_fdpic.c b/fs/binfmt_elf_fdpic.c index 38502c6..79d2b1a 100644 --- a/fs/binfmt_elf_fdpic.c +++ b/fs/binfmt_elf_fdpic.c @@ -380,7 +380,8 @@ static int load_elf_fdpic_binary(struct linux_binprm *bprm, down_write(¤t->mm->mmap_sem); current->mm->start_brk = do_mmap(NULL, 0, stack_size, PROT_READ | PROT_WRITE | PROT_EXEC, - MAP_PRIVATE | MAP_ANONYMOUS | MAP_GROWSDOWN, + MAP_PRIVATE | MAP_ANONYMOUS | + MAP_UNINITIALIZED | MAP_GROWSDOWN, 0); if (IS_ERR_VALUE(current->mm->start_brk)) { diff --git a/include/asm-generic/mman-common.h b/include/asm-generic/mman-common.h index 5ee13b2..2011126 100644 --- a/include/asm-generic/mman-common.h +++ b/include/asm-generic/mman-common.h @@ -19,6 +19,11 @@ #define MAP_TYPE 0x0f /* Mask for type of mapping */ #define MAP_FIXED 0x10 /* Interpret addr exactly */ #define MAP_ANONYMOUS 0x20 /* don't use a file */ +#ifdef CONFIG_MMAP_ALLOW_UNINITIALIZED +# define MAP_UNINITIALIZED 0x4000000 /* For anonymous mmap, memory could be uninitialized */ +#else +# define MAP_UNINITIALIZED 0x0 /* Don't support this flag */ +#endif #define MS_ASYNC 1 /* sync memory asynchronously */ #define MS_INVALIDATE 2 /* invalidate the caches */ diff --git a/init/Kconfig b/init/Kconfig index 54c655c..a23da9f 100644 --- a/init/Kconfig +++ b/init/Kconfig @@ -1079,6 +1079,28 @@ config SLOB endchoice +config MMAP_ALLOW_UNINITIALIZED + bool "Allow mmapped anonymous memory to be uninitialized" + depends on EMBEDDED && !MMU + default n + help + Normally, and according to the Linux spec, anonymous memory obtained + from mmap() has it's contents cleared before it is passed to + userspace. Enabling this config option allows you to request that + mmap() skip that if it is given an MAP_UNINITIALIZED flag, thus + providing a huge performance boost. If this option is not enabled, + then the flag will be ignored. + + This is taken advantage of by uClibc's malloc(), and also by + ELF-FDPIC binfmt's brk and stack allocator. + + Because of the obvious security issues, this option should only be + enabled on embedded devices where you control what is run in + userspace. Since that isn't generally a problem on no-MMU systems, + it is normally safe to say Y here. + + See Documentation/nommu-mmap.txt for more information. + config PROFILING bool "Profiling support (EXPERIMENTAL)" help @@ -1143,9 +1143,6 @@ static int do_mmap_private(struct vm_area_struct *vma, if (ret < rlen) memset(base + ret, 0, rlen - ret); - } else { - /* if it's an anonymous mapping, then just clear it */ - memset(base, 0, rlen); } return 0; @@ -1343,6 +1340,11 @@ unsigned long do_mmap_pgoff(struct file *file, goto error_just_free; add_nommu_region(region); + /* clear anonymous mappings that don't ask for uninitialized data */ + if (!vma->vm_file && !(flags & MAP_UNINITIALIZED)) + memset((void *)region->vm_start, 0, + region->vm_end - region->vm_start); + /* okay... we have a mapping; now we have to register it */ result = vma->vm_start; |
