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-rw-r--r--Documentation/nommu-mmap.txt26
-rw-r--r--fs/binfmt_elf_fdpic.c3
-rw-r--r--include/asm-generic/mman-common.h5
-rw-r--r--init/Kconfig22
-rw-r--r--mm/nommu.c8
5 files changed, 60 insertions, 4 deletions
diff --git a/Documentation/nommu-mmap.txt b/Documentation/nommu-mmap.txt
index b565e8279d13..8e1ddec2c78a 100644
--- a/Documentation/nommu-mmap.txt
+++ b/Documentation/nommu-mmap.txt
@@ -119,6 +119,32 @@ FURTHER NOTES ON NO-MMU MMAP
119 granule but will only discard the excess if appropriately configured as 119 granule but will only discard the excess if appropriately configured as
120 this has an effect on fragmentation. 120 this has an effect on fragmentation.
121 121
122 (*) The memory allocated by a request for an anonymous mapping will normally
123 be cleared by the kernel before being returned in accordance with the
124 Linux man pages (ver 2.22 or later).
125
126 In the MMU case this can be achieved with reasonable performance as
127 regions are backed by virtual pages, with the contents only being mapped
128 to cleared physical pages when a write happens on that specific page
129 (prior to which, the pages are effectively mapped to the global zero page
130 from which reads can take place). This spreads out the time it takes to
131 initialize the contents of a page - depending on the write-usage of the
132 mapping.
133
134 In the no-MMU case, however, anonymous mappings are backed by physical
135 pages, and the entire map is cleared at allocation time. This can cause
136 significant delays during a userspace malloc() as the C library does an
137 anonymous mapping and the kernel then does a memset for the entire map.
138
139 However, for memory that isn't required to be precleared - such as that
140 returned by malloc() - mmap() can take a MAP_UNINITIALIZED flag to
141 indicate to the kernel that it shouldn't bother clearing the memory before
142 returning it. Note that CONFIG_MMAP_ALLOW_UNINITIALIZED must be enabled
143 to permit this, otherwise the flag will be ignored.
144
145 uClibc uses this to speed up malloc(), and the ELF-FDPIC binfmt uses this
146 to allocate the brk and stack region.
147
122 (*) A list of all the private copy and anonymous mappings on the system is 148 (*) A list of all the private copy and anonymous mappings on the system is
123 visible through /proc/maps in no-MMU mode. 149 visible through /proc/maps in no-MMU mode.
124 150
diff --git a/fs/binfmt_elf_fdpic.c b/fs/binfmt_elf_fdpic.c
index 38502c67987c..79d2b1aa389f 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,
380 down_write(&current->mm->mmap_sem); 380 down_write(&current->mm->mmap_sem);
381 current->mm->start_brk = do_mmap(NULL, 0, stack_size, 381 current->mm->start_brk = do_mmap(NULL, 0, stack_size,
382 PROT_READ | PROT_WRITE | PROT_EXEC, 382 PROT_READ | PROT_WRITE | PROT_EXEC,
383 MAP_PRIVATE | MAP_ANONYMOUS | MAP_GROWSDOWN, 383 MAP_PRIVATE | MAP_ANONYMOUS |
384 MAP_UNINITIALIZED | MAP_GROWSDOWN,
384 0); 385 0);
385 386
386 if (IS_ERR_VALUE(current->mm->start_brk)) { 387 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 5ee13b2fd223..20111265afd8 100644
--- a/include/asm-generic/mman-common.h
+++ b/include/asm-generic/mman-common.h
@@ -19,6 +19,11 @@
19#define MAP_TYPE 0x0f /* Mask for type of mapping */ 19#define MAP_TYPE 0x0f /* Mask for type of mapping */
20#define MAP_FIXED 0x10 /* Interpret addr exactly */ 20#define MAP_FIXED 0x10 /* Interpret addr exactly */
21#define MAP_ANONYMOUS 0x20 /* don't use a file */ 21#define MAP_ANONYMOUS 0x20 /* don't use a file */
22#ifdef CONFIG_MMAP_ALLOW_UNINITIALIZED
23# define MAP_UNINITIALIZED 0x4000000 /* For anonymous mmap, memory could be uninitialized */
24#else
25# define MAP_UNINITIALIZED 0x0 /* Don't support this flag */
26#endif
22 27
23#define MS_ASYNC 1 /* sync memory asynchronously */ 28#define MS_ASYNC 1 /* sync memory asynchronously */
24#define MS_INVALIDATE 2 /* invalidate the caches */ 29#define MS_INVALIDATE 2 /* invalidate the caches */
diff --git a/init/Kconfig b/init/Kconfig
index 54c655ce9c04..a23da9f01803 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1079,6 +1079,28 @@ config SLOB
1079 1079
1080endchoice 1080endchoice
1081 1081
1082config MMAP_ALLOW_UNINITIALIZED
1083 bool "Allow mmapped anonymous memory to be uninitialized"
1084 depends on EMBEDDED && !MMU
1085 default n
1086 help
1087 Normally, and according to the Linux spec, anonymous memory obtained
1088 from mmap() has it's contents cleared before it is passed to
1089 userspace. Enabling this config option allows you to request that
1090 mmap() skip that if it is given an MAP_UNINITIALIZED flag, thus
1091 providing a huge performance boost. If this option is not enabled,
1092 then the flag will be ignored.
1093
1094 This is taken advantage of by uClibc's malloc(), and also by
1095 ELF-FDPIC binfmt's brk and stack allocator.
1096
1097 Because of the obvious security issues, this option should only be
1098 enabled on embedded devices where you control what is run in
1099 userspace. Since that isn't generally a problem on no-MMU systems,
1100 it is normally safe to say Y here.
1101
1102 See Documentation/nommu-mmap.txt for more information.
1103
1082config PROFILING 1104config PROFILING
1083 bool "Profiling support (EXPERIMENTAL)" 1105 bool "Profiling support (EXPERIMENTAL)"
1084 help 1106 help
diff --git a/mm/nommu.c b/mm/nommu.c
index 9876fa0c3ad3..8687973462bb 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -1143,9 +1143,6 @@ static int do_mmap_private(struct vm_area_struct *vma,
1143 if (ret < rlen) 1143 if (ret < rlen)
1144 memset(base + ret, 0, rlen - ret); 1144 memset(base + ret, 0, rlen - ret);
1145 1145
1146 } else {
1147 /* if it's an anonymous mapping, then just clear it */
1148 memset(base, 0, rlen);
1149 } 1146 }
1150 1147
1151 return 0; 1148 return 0;
@@ -1343,6 +1340,11 @@ unsigned long do_mmap_pgoff(struct file *file,
1343 goto error_just_free; 1340 goto error_just_free;
1344 add_nommu_region(region); 1341 add_nommu_region(region);
1345 1342
1343 /* clear anonymous mappings that don't ask for uninitialized data */
1344 if (!vma->vm_file && !(flags & MAP_UNINITIALIZED))
1345 memset((void *)region->vm_start, 0,
1346 region->vm_end - region->vm_start);
1347
1346 /* okay... we have a mapping; now we have to register it */ 1348 /* okay... we have a mapping; now we have to register it */
1347 result = vma->vm_start; 1349 result = vma->vm_start;
1348 1350