| Message ID | 20241210213050.2839638-4-bgeffon@google.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | mm: Fix mremap behavior when using addr hints | expand |
On Tue, Dec 10, 2024 at 1:31 PM Brian Geffon <bgeffon@google.com> wrote: > > When using MREMAP_MAYMOVE previously the new_addr was ignored unless > the user specified MREMAP_FIXED. This change will allow it to be > used as a hint in that situation similar to how mmap(2) behaves. > > get_unmapped_area() will handle page aligning the new address hint. > > Signed-off-by: Brian Geffon <bgeffon@google.com> > --- > mm/mremap.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/mm/mremap.c b/mm/mremap.c > index fdc1b0f1b38e..1d2522fba0ef 100644 > --- a/mm/mremap.c > +++ b/mm/mremap.c > @@ -1205,7 +1205,7 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len, > if (vma->vm_flags & VM_MAYSHARE) > map_flags |= MAP_SHARED; > > - new_addr = get_unmapped_area(vma->vm_file, 0, new_len, > + new_addr = get_unmapped_area(vma->vm_file, new_addr, new_len, Marco raised the concern that since glibc [1] handles new_addr as a variadic argument, existing call sites which used only MREMAP_MAYMOVE might end up passing garbage to the glibc wrapper for new_addr. I checked and it turns out that musl does the same [2]. So I'm not sure how this could ever be safely implemented? Worst case scenario we're leaking stack data as a hint to mremap, yikes. 1. https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/mremap.c 2. https://git.musl-libc.org/cgit/musl/tree/src/mman/mremap.c > vma->vm_pgoff + > ((addr - vma->vm_start) >> PAGE_SHIFT), > map_flags); > -- > 2.47.0.338.g60cca15819-goog >
diff --git a/mm/mremap.c b/mm/mremap.c index fdc1b0f1b38e..1d2522fba0ef 100644 --- a/mm/mremap.c +++ b/mm/mremap.c @@ -1205,7 +1205,7 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len, if (vma->vm_flags & VM_MAYSHARE) map_flags |= MAP_SHARED; - new_addr = get_unmapped_area(vma->vm_file, 0, new_len, + new_addr = get_unmapped_area(vma->vm_file, new_addr, new_len, vma->vm_pgoff + ((addr - vma->vm_start) >> PAGE_SHIFT), map_flags);
When using MREMAP_MAYMOVE previously the new_addr was ignored unless the user specified MREMAP_FIXED. This change will allow it to be used as a hint in that situation similar to how mmap(2) behaves. get_unmapped_area() will handle page aligning the new address hint. Signed-off-by: Brian Geffon <bgeffon@google.com> --- mm/mremap.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)