[Xen-devel] Re: [RFC, PATCH 7/24] i386 Vmi memory hole

[Zachary Amsden <zach@xxxxxxxxxx>]

Chris Wright wrote:
> * Zachary Amsden (zach@xxxxxxxxxx) wrote:
>   
> > Create a configurable hole in the linear address space at the top
> > of memory.  A more advanced interface is needed to negotiate how
> > much space the hypervisor is allowed to steal, but in the end, it
> > seems most likely that a fixed constant size will be chosen for
> > the compiled kernel, potentially propagated to an information
> > page used by paravirtual initialization to determine interface
> > compatibility.
> >
> > Signed-off-by: Zachary Amsden <zach@xxxxxxxxxx>
> >
> > Index: linux-2.6.16-rc3/arch/i386/Kconfig
> > ===================================================================
> > --- linux-2.6.16-rc3.orig/arch/i386/Kconfig     2006-02-22 16:09:04.000000000 
> > -0800
> > +++ linux-2.6.16-rc3/arch/i386/Kconfig  2006-02-22 16:33:27.000000000 -0800
> > @@ -201,6 +201,15 @@ config VMI_DEBUG
> >  
> >  endmenu
> >  
> > +config MEMORY_HOLE
> > +       int "Create hole at top of memory (0-256 MB)"
> > +       range 0 256
> > +       default "64" if X86_VMI
> > +       default "0" if !X86_VMI
> >     
> Deja-vu ;-)  And still works in context of Xen, but we've just let the
> subarch define the __FIXADDR_TOP.  Having it be dynamic could be
> interesting.
>   
Here's dynamic.  I hope it still applies.

Allow creation of an compile time hole at the top of linear address space.

Extended to allow a dynamic hole in linear address space, 7/2005.  This
required some serious hacking to get everything perfect, but the end result
appears to function quite nicely.  Everyone can now share the appreciation
of pseudo-undocumented ELF OS fields, which means core dumps, debuggers
and even broken or obsolete linkers may continue to work.

Signed-off-by: Zachary Amsden <zach@xxxxxxxxxx>
Index: linux-2.6.13/arch/i386/Kconfig
===================================================================
--- linux-2.6.13.orig/arch/i386/Kconfig 2005-08-04 14:14:24.000000000 -0700
+++ linux-2.6.13/arch/i386/Kconfig      2005-08-05 15:28:42.000000000 -0700
@@ -127,6 +127,20 @@
 
 endchoice
 
+config RELOCATABLE_FIXMAP
+       bool "Allow the fixmap to be placed dynamically at runtime"
+       depends on EXPERIMENTAL
+       help
+         Crazy hackers only.
+
+config MEMORY_HOLE
+       int "Create hole at top of memory (0-512 MB)"
+       range 0 512
+       default "0"
+       help
+         Useful for creating a hole in the top of memory when running
+         inside of a virtual machine monitor.
+
 config ACPI_SRAT
        bool
        default y
Index: linux-2.6.13/arch/i386/kernel/sysenter.c
===================================================================
--- linux-2.6.13.orig/arch/i386/kernel/sysenter.c       2005-08-02 
17:04:12.000000000 -0700
+++ linux-2.6.13/arch/i386/kernel/sysenter.c    2005-08-05 15:47:53.000000000 
-0700
@@ -46,22 +46,90 @@
 extern const char vsyscall_int80_start, vsyscall_int80_end;
 extern const char vsyscall_sysenter_start, vsyscall_sysenter_end;
 
+#ifdef CONFIG_RELOCATABLE_FIXMAP
+extern const char SYSENTER_RETURN;
+const char *SYSENTER_RETURN_ADDR;
+
+static void fixup_vsyscall_elf(char *page)
+{
+       Elf32_Ehdr *hdr;
+       Elf32_Shdr *sechdrs;
+       Elf32_Phdr *phdr;
+       char *secstrings;
+       int i, j, n;
+
+       hdr = (Elf32_Ehdr *)page;
+
+       /* Sanity checks against insmoding binaries or wrong arch,
+           weird elf version */
+       if (memcmp(hdr->e_ident, ELFMAG, 4) != 0 ||
+               !elf_check_arch(hdr) ||
+               hdr->e_type != ET_DYN)
+               panic("Bogus ELF in vsyscall DSO\n");
+
+       hdr->e_entry += VSYSCALL_RELOCATION;
+
+       sechdrs = (void *)hdr + hdr->e_shoff;
+       secstrings = (void *)hdr + sechdrs[hdr->e_shstrndx].sh_offset;
+
+       for (i = 1; i < hdr->e_shnum; i++) {
+               if (!(sechdrs[i].sh_flags & SHF_ALLOC))
+                       continue;
+
+               sechdrs[i].sh_addr += VSYSCALL_RELOCATION;
+               if (strcmp(secstrings+sechdrs[i].sh_name, ".dynsym") == 0) {
+                       Elf32_Sym  *sym =  (void *)hdr + sechdrs[i].sh_offset;
+                       n = sechdrs[i].sh_size / sizeof(*sym);
+                       for (j = 1; j < n;  j++) {
+                               int ndx = sym[j].st_shndx;
+                               if (ndx == SHN_UNDEF || ndx == SHN_ABS)
+                                       continue;
+                               sym[j].st_value += VSYSCALL_RELOCATION;
+                       }
+               } else if (strcmp(secstrings+sechdrs[i].sh_name, ".dynamic") == 
0) {
+                       Elf32_Dyn *dyn = (void *)hdr + sechdrs[i].sh_offset;
+                       int tag;
+                       while ((tag = (++dyn)->d_tag) != DT_NULL) {
+                               if (tag == DT_PLTGOT || tag == DT_HASH ||
+                                   tag == DT_STRTAB || tag == DT_SYMTAB ||
+                                   tag == DT_RELA || tag == DT_INIT ||
+                                   tag == DT_FINI || tag == DT_REL ||
+                                   tag == DT_JMPREL || tag == DT_VERSYM ||
+                                   tag == DT_VERDEF || tag == DT_VERNEED)
+                                       dyn->d_un.d_val += VSYSCALL_RELOCATION;
+                       }
+               } else if (strcmp(secstrings+sechdrs[i].sh_name, ".useless") == 
0) {
+                       uint32_t *got = (void *)hdr + sechdrs[i].sh_offset;
+                       *got += VSYSCALL_RELOCATION;
+               }
+       }
+       phdr = (void *)hdr + hdr->e_phoff;
+       for (i = 0; i < hdr->e_phnum; i++) {
+               phdr[i].p_vaddr += VSYSCALL_RELOCATION;
+               phdr[i].p_paddr += VSYSCALL_RELOCATION;
+       }
+       SYSENTER_RETURN_ADDR = (char *)&SYSENTER_RETURN + VSYSCALL_RELOCATION;
+}
+#endif
+
 int __init sysenter_setup(void)
 {
        void *page = (void *)get_zeroed_page(GFP_ATOMIC);
 
-       __set_fixmap(FIX_VSYSCALL, __pa(page), PAGE_READONLY_EXEC);
-
-       if (!boot_cpu_has(X86_FEATURE_SEP)) {
+       if (!boot_cpu_has(X86_FEATURE_SEP))
                memcpy(page,
                       &vsyscall_int80_start,
                       &vsyscall_int80_end - &vsyscall_int80_start);
-               return 0;
-       }
+       else
+               memcpy(page,
+                       &vsyscall_sysenter_start,
+                       &vsyscall_sysenter_end - &vsyscall_sysenter_start);
 
-       memcpy(page,
-              &vsyscall_sysenter_start,
-              &vsyscall_sysenter_end - &vsyscall_sysenter_start);
+#ifdef CONFIG_RELOCATABLE_FIXMAP
+       fixup_vsyscall_elf((char *)page);
+#endif
+
+       __set_fixmap(FIX_VSYSCALL, __pa(page), PAGE_READONLY_EXEC);
 
        return 0;
 }
Index: linux-2.6.13/arch/i386/kernel/asm-offsets.c
===================================================================
--- linux-2.6.13.orig/arch/i386/kernel/asm-offsets.c    2005-08-04 
14:28:35.000000000 -0700
+++ linux-2.6.13/arch/i386/kernel/asm-offsets.c 2005-08-05 15:11:45.000000000 
-0700
@@ -68,5 +68,9 @@
                 sizeof(struct tss_struct));
 
        DEFINE(PAGE_SIZE_asm, PAGE_SIZE);
+#ifdef CONFIG_RELOCATABLE_FIXMAP
+       DEFINE(VSYSCALL_BASE, 0);
+#else
        DEFINE(VSYSCALL_BASE, __fix_to_virt(FIX_VSYSCALL));
+#endif
 }
Index: linux-2.6.13/arch/i386/kernel/signal.c
===================================================================
--- linux-2.6.13.orig/arch/i386/kernel/signal.c 2005-08-03 23:36:46.000000000 
-0700
+++ linux-2.6.13/arch/i386/kernel/signal.c      2005-08-05 15:11:33.000000000 
-0700
@@ -345,6 +345,8 @@
    See vsyscall-sigreturn.S.  */
 extern void __user __kernel_sigreturn;
 extern void __user __kernel_rt_sigreturn;
+#define kernel_sigreturn  (VSYSCALL_RELOCATION + (void __user 
*)&__kernel_sigreturn)
+#define kernel_rt_sigreturn  (VSYSCALL_RELOCATION + (void __user 
*)&__kernel_rt_sigreturn)
 
 static int setup_frame(int sig, struct k_sigaction *ka,
                       sigset_t *set, struct pt_regs * regs)
@@ -380,7 +382,7 @@
                        goto give_sigsegv;
        }
 
-       restorer = &__kernel_sigreturn;
+       restorer = kernel_sigreturn;
        if (ka->sa.sa_flags & SA_RESTORER)
                restorer = ka->sa.sa_restorer;
 
@@ -476,7 +478,7 @@
                goto give_sigsegv;
 
        /* Set up to return from userspace.  */
-       restorer = &__kernel_rt_sigreturn;
+       restorer = kernel_rt_sigreturn;
        if (ka->sa.sa_flags & SA_RESTORER)
                restorer = ka->sa.sa_restorer;
        err |= __put_user(restorer, &frame->pretcode);
Index: linux-2.6.13/arch/i386/kernel/entry.S
===================================================================
--- linux-2.6.13.orig/arch/i386/kernel/entry.S  2005-08-04 14:17:15.000000000 
-0700
+++ linux-2.6.13/arch/i386/kernel/entry.S       2005-08-05 14:09:15.000000000 
-0700
@@ -200,7 +200,11 @@
        pushl %ebp
        pushfl
        pushl $(__USER_CS)
+#ifdef CONFIG_RELOCATABLE_FIXMAP
+       pushl %ss:SYSENTER_RETURN_ADDR
+#else
        pushl $SYSENTER_RETURN
+#endif
 
 /*
  * Load the potential sixth argument from user stack.
Index: linux-2.6.13/arch/i386/mm/init.c
===================================================================
--- linux-2.6.13.orig/arch/i386/mm/init.c       2005-08-04 14:39:17.000000000 
-0700
+++ linux-2.6.13/arch/i386/mm/init.c    2005-08-05 15:20:04.000000000 -0700
@@ -42,6 +42,10 @@
 
 unsigned int __VMALLOC_RESERVE = 128 << 20;
 
+#ifdef CONFIG_RELOCATABLE_FIXMAP
+unsigned long __FIXADDR_TOP = 0;
+#endif
+
 DEFINE_PER_CPU(struct mmu_gather, mmu_gathers);
 unsigned long highstart_pfn, highend_pfn;
 
@@ -478,6 +482,12 @@
                printk("NX (Execute Disable) protection: active\n");
 #endif
 
+#ifdef CONFIG_RELOCATABLE_FIXMAP
+       if (!__FIXADDR_TOP) 
+               __FIXADDR_TOP =  0xfffff000UL-(CONFIG_MEMORY_HOLE << 20);
+       printk(KERN_INFO "Fixmap top relocated to %lxh\n", __FIXADDR_TOP);
+#endif
+
        pagetable_init();
 
        load_cr3(swapper_pg_dir);
Index: linux-2.6.13/include/asm-i386/fixmap.h
===================================================================
--- linux-2.6.13.orig/include/asm-i386/fixmap.h 2005-08-04 14:14:24.000000000 
-0700
+++ linux-2.6.13/include/asm-i386/fixmap.h      2005-08-05 15:36:13.000000000 
-0700
@@ -20,7 +20,13 @@
  * Leave one empty page between vmalloc'ed areas and
  * the start of the fixmap.
  */
-#define __FIXADDR_TOP  0xfffff000
+#ifdef CONFIG_RELOCATABLE_FIXMAP
+extern unsigned long __FIXADDR_TOP;
+#define VSYSCALL_RELOCATION __fix_to_virt(FIX_VSYSCALL)
+#else
+#define __FIXADDR_TOP  (0xfffff000-(CONFIG_MEMORY_HOLE << 20))
+#define VSYSCALL_RELOCATION 0
+#endif
 
 #ifndef __ASSEMBLY__
 #include <linux/kernel.h>
Index: linux-2.6.13/include/asm-i386/elf.h
===================================================================
--- linux-2.6.13.orig/include/asm-i386/elf.h    2005-08-02 17:06:23.000000000 
-0700
+++ linux-2.6.13/include/asm-i386/elf.h 2005-08-05 15:31:32.000000000 -0700
@@ -129,7 +129,7 @@
 
 #define VSYSCALL_BASE  (__fix_to_virt(FIX_VSYSCALL))
 #define VSYSCALL_EHDR  ((const struct elfhdr *) VSYSCALL_BASE)
-#define VSYSCALL_ENTRY ((unsigned long) &__kernel_vsyscall)
+#define VSYSCALL_ENTRY ((unsigned long) 
(VSYSCALL_RELOCATION+&__kernel_vsyscall))
 extern void __kernel_vsyscall;
 
 #define ARCH_DLINFO                                            \
Index: linux-2.6.13/include/linux/elf.h
===================================================================
--- linux-2.6.13.orig/include/linux/elf.h       2005-08-02 17:06:24.000000000 
-0700
+++ linux-2.6.13/include/linux/elf.h    2005-08-05 12:06:17.000000000 -0700
@@ -138,6 +138,9 @@
 #define DT_DEBUG       21
 #define DT_TEXTREL     22
 #define DT_JMPREL      23
+#define DT_VERSYM      0x6ffffff0
+#define DT_VERDEF      0x6ffffffc
+#define DT_VERNEED     0x6ffffffe
 #define DT_LOPROC      0x70000000
 #define DT_HIPROC      0x7fffffff
 

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