| Message ID | 20240925210615.2572360-6-arnd@kernel.org (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | asm-generic: clean up asm/mman.h | expand |
On 25.09.24 23:06, Arnd Bergmann wrote: > From: Arnd Bergmann <arnd@arndb.de> > > MAP_UNINITIALIZED was added back in 2009 for NOMMU kernels, specifically > for blackfin, which is long gone. MAP_HUGE_SHIFT/MAP_HUGE_MASK were > added in 2012 for architectures supporting hugepages, which at the time > did not overlap with the ones supporting NOMMU. > > Adding the macro under an #ifdef was obviously a mistake, which > Christoph Hellwig tried to address by making it unconditionally defined > to 0x4000000 as part of the series to support RISC-V NOMMU kernels. At > this point linux/mman.h contained two conflicting definitions for bit 26, > though the two are still mutually exclusive at runtime in all supported > configurations. > > According to the commit 854e9ed09ded ("mm: support madvise(MADV_FREE)") > description, it was previously used internally by facebook, which > would have resulted in MAP_HUGE_1MB turning into MAP_HUGE_2MB > with MAP_UNINITIALIZED enabled, and every other page size implying > MAP_UNINITIALIZED. I assume there are no remaining out of tree users > on MMU-enabled kernels today. > > I do not see any sensible way to redefine the macros for the ABI in > a way avoids breaking something. The only ideas so far are: > > - do nothing, try to document the bug, hope for the best > > - remove the kernel implementation and redefine MAP_UNINITIALIZED to > zero in the header to silently turn it off for everyone. There are > few NOMMU users left, and the ones that do use NOMMU usually turn > off MMAP_ALLOW_UNINITIALIZED, as it still has the potential to cause > bugs and even security issues on systems with a memory protection > unit. > > - remove both the implementation and the macro to force a build > failure for anyone trying to use the feature. This way we can > see who complains and whether we need to put it back in some > form or change the userspace sources to no longer pass the flag. > The first, uncontroversial step could indeed be to make MAP_UNINITIALIZED a nop, but still leave the definitions in mman.h etc around. This is the same we did with MAP_DENYWRITE. There might be some weird user out there, and carelessly reusing the bit could result in trouble. (people might argue that they are not using it with MAP_HUGETLB, so it would work) Going forward and removing MAP_UNINITIALIZED is a bit more controversial, but maybe there really isn't any other user around. Software that is not getting recompiled cannot be really identified by letting it rest in -next only. My take would be to leave MAP_UNINITIALIZED in the headers in some form for documentation purposes.
On 9/25/24 23:06, Arnd Bergmann wrote: > From: Arnd Bergmann <arnd@arndb.de> > > MAP_UNINITIALIZED was added back in 2009 for NOMMU kernels, specifically > for blackfin, which is long gone. MAP_HUGE_SHIFT/MAP_HUGE_MASK were > added in 2012 for architectures supporting hugepages, which at the time > did not overlap with the ones supporting NOMMU. > > Adding the macro under an #ifdef was obviously a mistake, which > Christoph Hellwig tried to address by making it unconditionally defined > to 0x4000000 as part of the series to support RISC-V NOMMU kernels. At > this point linux/mman.h contained two conflicting definitions for bit 26, > though the two are still mutually exclusive at runtime in all supported > configurations. > > According to the commit 854e9ed09ded ("mm: support madvise(MADV_FREE)") > description, it was previously used internally by facebook, which > would have resulted in MAP_HUGE_1MB turning into MAP_HUGE_2MB > with MAP_UNINITIALIZED enabled, and every other page size implying > MAP_UNINITIALIZED. I assume there are no remaining out of tree users > on MMU-enabled kernels today. > > I do not see any sensible way to redefine the macros for the ABI in > a way avoids breaking something. The only ideas so far are: > > - do nothing, try to document the bug, hope for the best > > - remove the kernel implementation and redefine MAP_UNINITIALIZED to > zero in the header to silently turn it off for everyone. There are > few NOMMU users left, and the ones that do use NOMMU usually turn > off MMAP_ALLOW_UNINITIALIZED, as it still has the potential to cause > bugs and even security issues on systems with a memory protection > unit. > > - remove both the implementation and the macro to force a build > failure for anyone trying to use the feature. This way we can > see who complains and whether we need to put it back in some > form or change the userspace sources to no longer pass the flag. > > Implement the third option here for the sake of discussion. Usually I'd vote for option #2, which means remove the kernel implementation and redefine MAP_UNINITIALIZED to zero in the header. A few years back this turned out to be the "most compatible" solution. But today, and specifically regarding MAP_UNINITIALIZED, I think we should get rid of it now. This flag is useless and build issues will force people to drop it. Helge
On Thu, Sep 26, 2024, at 08:46, David Hildenbrand wrote: > On 25.09.24 23:06, Arnd Bergmann wrote: > > The first, uncontroversial step could indeed be to make > MAP_UNINITIALIZED a nop, but still leave the definitions in mman.h etc > around. > > This is the same we did with MAP_DENYWRITE. There might be some weird > user out there, and carelessly reusing the bit could result in trouble. > (people might argue that they are not using it with MAP_HUGETLB, so it > would work) > > Going forward and removing MAP_UNINITIALIZED is a bit more > controversial, but maybe there really isn't any other user around. > Software that is not getting recompiled cannot be really identified by > letting it rest in -next only. > > My take would be to leave MAP_UNINITIALIZED in the headers in some form > for documentation purposes. I don't think there is much point in doing this in multiple steps, either we want to break it at compile time or leave it silently doing nothing. There is also very little difference in practice because applications almost always use sys/mman.h instead of linux/mman.h. FWIW, the main user appears to be the uClibc and uclibc-ng malloc() implementation for NOMMU targets: https://git.uclibc.org/uClibc/commit/libc/stdlib/malloc/malloc.c?id=00673f93826bf1f Both of these also define this constant itself as 0x4000000 for all architectures. There are a few others that I could find with Debian codesearch: https://sources.debian.org/src/monado/21.0.0+git2905.e26a272c1~dfsg1-2/src/external/tracy/client/tracy_rpmalloc.cpp/?hl=890#L889 https://sources.debian.org/src/systemtap/5.1-4/testsuite/systemtap.syscall/mmap.c/?hl=224#L224 https://sources.debian.org/src/fuzzel/1.11.1+ds-1/shm.c/?hl=488#L488 https://sources.debian.org/src/notcurses/3.0.7+dfsg.1-1/src/lib/fbuf.h/?hl=35#L35 https://sources.debian.org/src/lmms/1.2.2+dfsg1-6/src/3rdparty/rpmalloc/rpmalloc/rpmalloc/rpmalloc.c/?hl=1753#L1753 All of these will fall back to not passing MAP_UNINITIALIZED if it's not defined, which is what happens on glibc and musl. Arnd
diff --git a/Documentation/admin-guide/mm/nommu-mmap.rst b/Documentation/admin-guide/mm/nommu-mmap.rst index 530fed08de2c..9434c2fa99ae 100644 --- a/Documentation/admin-guide/mm/nommu-mmap.rst +++ b/Documentation/admin-guide/mm/nommu-mmap.rst @@ -135,14 +135,8 @@ Further notes on no-MMU MMAP 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. + Previously, Linux also supported a MAP_UNINITIALIZED flag to allocate + memory without clearing it, this is no longer support. (#) 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/arch/alpha/include/uapi/asm/mman.h b/arch/alpha/include/uapi/asm/mman.h index fc8b74aa3f89..1099b17a4003 100644 --- a/arch/alpha/include/uapi/asm/mman.h +++ b/arch/alpha/include/uapi/asm/mman.h @@ -21,8 +21,6 @@ /* MAP_SYNC not supported */ #define MAP_FIXED_NOREPLACE 0x200000/* MAP_FIXED which doesn't unmap underlying mapping */ -/* MAP_UNINITIALIZED not supported */ - /* * Flags for mlockall */ diff --git a/arch/mips/include/uapi/asm/mman.h b/arch/mips/include/uapi/asm/mman.h index 6deb62db90de..9463c9071268 100644 --- a/arch/mips/include/uapi/asm/mman.h +++ b/arch/mips/include/uapi/asm/mman.h @@ -31,8 +31,6 @@ /* MAP_SYNC not supported */ #define MAP_FIXED_NOREPLACE 0x100000 /* MAP_FIXED which doesn't unmap underlying mapping */ -/* MAP_UNINITIALIZED not supported */ - /* * Flags for mlockall */ diff --git a/arch/parisc/include/uapi/asm/mman.h b/arch/parisc/include/uapi/asm/mman.h index 3732950a5cd8..8d7f3a8912b3 100644 --- a/arch/parisc/include/uapi/asm/mman.h +++ b/arch/parisc/include/uapi/asm/mman.h @@ -20,8 +20,6 @@ /* MAP_SYNC not supported */ #define MAP_FIXED_NOREPLACE 0x100000 /* MAP_FIXED which doesn't unmap underlying mapping */ -/* MAP_UNINITIALIZED not supported */ - /* * Flags for mlockall */ diff --git a/arch/powerpc/include/uapi/asm/mman.h b/arch/powerpc/include/uapi/asm/mman.h index d57b347c37fe..48c734b4d201 100644 --- a/arch/powerpc/include/uapi/asm/mman.h +++ b/arch/powerpc/include/uapi/asm/mman.h @@ -33,11 +33,6 @@ #define MAP_SYNC 0x080000 /* perform synchronous page faults for the mapping */ #define MAP_FIXED_NOREPLACE 0x100000 /* MAP_FIXED which doesn't unmap underlying mapping */ -#define MAP_UNINITIALIZED 0x4000000 /* For anonymous mmap, memory could be - * uninitialized */ - - - #define MCL_CURRENT 0x2000 /* lock all currently mapped pages */ #define MCL_FUTURE 0x4000 /* lock all additions to address space */ #define MCL_ONFAULT 0x8000 /* lock all pages that are faulted in */ diff --git a/arch/sh/configs/rsk7264_defconfig b/arch/sh/configs/rsk7264_defconfig index e4ef259425c4..86421e2fec10 100644 --- a/arch/sh/configs/rsk7264_defconfig +++ b/arch/sh/configs/rsk7264_defconfig @@ -12,7 +12,6 @@ CONFIG_KALLSYMS_ALL=y CONFIG_EXPERT=y CONFIG_PERF_COUNTERS=y # CONFIG_VM_EVENT_COUNTERS is not set -CONFIG_MMAP_ALLOW_UNINITIALIZED=y CONFIG_PROFILING=y # CONFIG_BLK_DEV_BSG is not set CONFIG_PARTITION_ADVANCED=y diff --git a/arch/sparc/include/uapi/asm/mman.h b/arch/sparc/include/uapi/asm/mman.h index afb86698cdb1..e05ac492f9a8 100644 --- a/arch/sparc/include/uapi/asm/mman.h +++ b/arch/sparc/include/uapi/asm/mman.h @@ -30,9 +30,6 @@ #define MAP_SYNC 0x080000 /* perform synchronous page faults for the mapping */ #define MAP_FIXED_NOREPLACE 0x100000 /* MAP_FIXED which doesn't unmap underlying mapping */ -#define MAP_UNINITIALIZED 0x4000000 /* For anonymous mmap, memory could be - * uninitialized */ - #define MCL_CURRENT 0x2000 /* lock all currently mapped pages */ #define MCL_FUTURE 0x4000 /* lock all additions to address space */ #define MCL_ONFAULT 0x8000 /* lock all pages that are faulted in */ diff --git a/arch/xtensa/include/uapi/asm/mman.h b/arch/xtensa/include/uapi/asm/mman.h index e713b8dc8587..6fdf9f3e587a 100644 --- a/arch/xtensa/include/uapi/asm/mman.h +++ b/arch/xtensa/include/uapi/asm/mman.h @@ -36,9 +36,6 @@ /* MAP_SYNC not supported */ #define MAP_FIXED_NOREPLACE 0x100000 /* MAP_FIXED which doesn't unmap underlying mapping */ -#define MAP_UNINITIALIZED 0x4000000 /* For anonymous mmap, memory could be - * uninitialized */ - /* * Flags for mlockall */ diff --git a/fs/binfmt_elf_fdpic.c b/fs/binfmt_elf_fdpic.c index 4fe5bb9f1b1f..82ba92d28ddf 100644 --- a/fs/binfmt_elf_fdpic.c +++ b/fs/binfmt_elf_fdpic.c @@ -418,8 +418,7 @@ static int load_elf_fdpic_binary(struct linux_binprm *bprm) current->mm->start_brk = vm_mmap(NULL, 0, stack_size, stack_prot, MAP_PRIVATE | MAP_ANONYMOUS | - MAP_UNINITIALIZED | MAP_GROWSDOWN, - 0); + MAP_GROWSDOWN, 0); if (IS_ERR_VALUE(current->mm->start_brk)) { retval = current->mm->start_brk; diff --git a/include/linux/mman.h b/include/linux/mman.h index bcb201ab7a41..f606b2264cc0 100644 --- a/include/linux/mman.h +++ b/include/linux/mman.h @@ -24,9 +24,6 @@ #ifndef MAP_HUGE_1GB #define MAP_HUGE_1GB 0 #endif -#ifndef MAP_UNINITIALIZED -#define MAP_UNINITIALIZED 0 -#endif #ifndef MAP_SYNC #define MAP_SYNC 0 #endif @@ -44,7 +41,6 @@ | MAP_ANONYMOUS \ | MAP_DENYWRITE \ | MAP_EXECUTABLE \ - | MAP_UNINITIALIZED \ | MAP_GROWSDOWN \ | MAP_LOCKED \ | MAP_NORESERVE \ diff --git a/include/uapi/asm-generic/mman.h b/include/uapi/asm-generic/mman.h index f26f9b4c03e1..541be26ad947 100644 --- a/include/uapi/asm-generic/mman.h +++ b/include/uapi/asm-generic/mman.h @@ -27,10 +27,6 @@ #define MAP_SYNC 0x080000 /* perform synchronous page faults for the mapping */ #define MAP_FIXED_NOREPLACE 0x100000 /* MAP_FIXED which doesn't unmap underlying mapping */ -#define MAP_UNINITIALIZED 0x4000000 /* For anonymous mmap, memory could be - * uninitialized */ - - /* * Bits [26:31] are reserved, see asm-generic/hugetlb_encode.h * for MAP_HUGETLB usage diff --git a/mm/Kconfig b/mm/Kconfig index 09aebca1cae3..7326820ba200 100644 --- a/mm/Kconfig +++ b/mm/Kconfig @@ -381,28 +381,6 @@ config COMPAT_BRK On non-ancient distros (post-2000 ones) N is usually a safe choice. -config MMAP_ALLOW_UNINITIALIZED - bool "Allow mmapped anonymous memory to be uninitialized" - depends on EXPERT && !MMU - default n - help - Normally, and according to the Linux spec, anonymous memory obtained - from mmap() has its 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/admin-guide/mm/nommu-mmap.rst for more information. - config SELECT_MEMORY_MODEL def_bool y depends on ARCH_SELECT_MEMORY_MODEL diff --git a/mm/nommu.c b/mm/nommu.c index 385b0c15add8..793fa7303065 100644 --- a/mm/nommu.c +++ b/mm/nommu.c @@ -1172,9 +1172,7 @@ unsigned long do_mmap(struct file *file, add_nommu_region(region); /* clear anonymous mappings that don't ask for uninitialized data */ - if (!vma->vm_file && - (!IS_ENABLED(CONFIG_MMAP_ALLOW_UNINITIALIZED) || - !(flags & MAP_UNINITIALIZED))) + if (!vma->vm_file) memset((void *)region->vm_start, 0, region->vm_end - region->vm_start);