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linux-user: completely re-write init_guest_space

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First we ensure all guest space initialisation logic comes through
probe_guest_base once we understand the nature of the binary we are
loading. The convoluted init_guest_space routine is removed and
replaced with a number of pgb_* helpers which are called depending on
what requirements we have when loading the binary.

We first try to do what is requested by the host. Failing that we try
and satisfy the guest requested base address. If all those options
fail we fall back to finding a space in the memory map using our
recently written read_self_maps() helper.

There are some additional complications we try and take into account
when looking for holes in the address space. We try not to go directly
after the system brk() space so there is space for a little growth. We
also don't want to have to use negative offsets which would result in
slightly less efficient code on x86 when it's unable to use the
segment offset register.

Less mind-binding gotos and hopefully clearer logic throughout.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Acked-by: Laurent Vivier <laurent@vivier.eu>

Message-Id: <20200513175134.19619-5-alex.bennee@linaro.org>
This commit is contained in:
Alex Bennée 2020-05-13 18:51:28 +01:00
parent aae8b87e9c
commit ee94743034
4 changed files with 303 additions and 312 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

555
linux-user/elfload.c
View File

@ -11,6 +11,7 @@
#include "qemu/queue.h" #include "qemu/queue.h"
#include "qemu/guest-random.h" #include "qemu/guest-random.h"
#include "qemu/units.h" #include "qemu/units.h"
#include "qemu/selfmap.h"
#ifdef _ARCH_PPC64 #ifdef _ARCH_PPC64
#undef ARCH_DLINFO #undef ARCH_DLINFO
@ -382,68 +383,30 @@ enum {
/* The commpage only exists for 32 bit kernels */ /* The commpage only exists for 32 bit kernels */
/* Return 1 if the proposed guest space is suitable for the guest. #define ARM_COMMPAGE (intptr_t)0xffff0f00u
* Return 0 if the proposed guest space isn't suitable, but another
* address space should be tried. static bool init_guest_commpage(void)
* Return -1 if there is no way the proposed guest space can be
* valid regardless of the base.
* The guest code may leave a page mapped and populate it if the
* address is suitable.
*/
static int init_guest_commpage(unsigned long guest_base,
unsigned long guest_size)
{ {
unsigned long real_start, test_page_addr; void *want = g2h(ARM_COMMPAGE & -qemu_host_page_size);
void *addr = mmap(want, qemu_host_page_size, PROT_READ | PROT_WRITE,
MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
/* We need to check that we can force a fault on access to the if (addr == MAP_FAILED) {
* commpage at 0xffff0fxx perror("Allocating guest commpage");
*/ exit(EXIT_FAILURE);
test_page_addr = guest_base + (0xffff0f00 & qemu_host_page_mask); }
if (addr != want) {
/* If the commpage lies within the already allocated guest space, return false;
* then there is no way we can allocate it.
*
* You may be thinking that that this check is redundant because
* we already validated the guest size against MAX_RESERVED_VA;
* but if qemu_host_page_mask is unusually large, then
* test_page_addr may be lower.
*/
if (test_page_addr >= guest_base
&& test_page_addr < (guest_base + guest_size)) {
return -1;
} }
/* Note it needs to be writeable to let us initialise it */ /* Set kernel helper versions; rest of page is 0. */
real_start = (unsigned long) __put_user(5, (uint32_t *)g2h(0xffff0ffcu));
mmap((void *)test_page_addr, qemu_host_page_size,
PROT_READ | PROT_WRITE,
MAP_ANONYMOUS | MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
/* If we can't map it then try another address */ if (mprotect(addr, qemu_host_page_size, PROT_READ)) {
if (real_start == -1ul) {
return 0;
}
if (real_start != test_page_addr) {
/* OS didn't put the page where we asked - unmap and reject */
munmap((void *)real_start, qemu_host_page_size);
return 0;
}
/* Leave the page mapped
* Populate it (mmap should have left it all 0'd)
*/
/* Kernel helper versions */
__put_user(5, (uint32_t *)g2h(0xffff0ffcul));
/* Now it's populated make it RO */
if (mprotect((void *)test_page_addr, qemu_host_page_size, PROT_READ)) {
perror("Protecting guest commpage"); perror("Protecting guest commpage");
exit(-1); exit(EXIT_FAILURE);
} }
return true;
return 1; /* All good */
} }
#define ELF_HWCAP get_elf_hwcap() #define ELF_HWCAP get_elf_hwcap()
@ -2075,240 +2038,268 @@ static abi_ulong create_elf_tables(abi_ulong p, int argc, int envc,
return sp; return sp;
} }
unsigned long init_guest_space(unsigned long host_start, #ifndef ARM_COMMPAGE
unsigned long host_size, #define ARM_COMMPAGE 0
unsigned long guest_start, #define init_guest_commpage() true
bool fixed) #endif
static void pgb_fail_in_use(const char *image_name)
{
error_report("%s: requires virtual address space that is in use "
"(omit the -B option or choose a different value)",
image_name);
exit(EXIT_FAILURE);
}
static void pgb_have_guest_base(const char *image_name, abi_ulong guest_loaddr,
abi_ulong guest_hiaddr, long align)
{
const int flags = MAP_ANONYMOUS | MAP_PRIVATE | MAP_NORESERVE;
void *addr, *test;
if (!QEMU_IS_ALIGNED(guest_base, align)) {
fprintf(stderr, "Requested guest base 0x%lx does not satisfy "
"host minimum alignment (0x%lx)\n",
guest_base, align);
exit(EXIT_FAILURE);
}
/* Sanity check the guest binary. */
if (reserved_va) {
if (guest_hiaddr > reserved_va) {
error_report("%s: requires more than reserved virtual "
"address space (0x%" PRIx64 " > 0x%lx)",
image_name, (uint64_t)guest_hiaddr, reserved_va);
exit(EXIT_FAILURE);
}
} else {
if ((guest_hiaddr - guest_base) > ~(uintptr_t)0) {
error_report("%s: requires more virtual address space "
"than the host can provide (0x%" PRIx64 ")",
image_name, (uint64_t)guest_hiaddr - guest_base);
exit(EXIT_FAILURE);
}
}
/*
* Expand the allocation to the entire reserved_va.
* Exclude the mmap_min_addr hole.
*/
if (reserved_va) {
guest_loaddr = (guest_base >= mmap_min_addr ? 0
: mmap_min_addr - guest_base);
guest_hiaddr = reserved_va;
}
/* Reserve the address space for the binary, or reserved_va. */
test = g2h(guest_loaddr);
addr = mmap(test, guest_hiaddr - guest_loaddr, PROT_NONE, flags, -1, 0);
if (test != addr) {
pgb_fail_in_use(image_name);
}
}
/* Return value for guest_base, or -1 if no hole found. */
static uintptr_t pgb_find_hole(uintptr_t guest_loaddr, uintptr_t guest_size,
long align)
{
GSList *maps, *iter;
uintptr_t this_start, this_end, next_start, brk;
intptr_t ret = -1;
assert(QEMU_IS_ALIGNED(guest_loaddr, align));
maps = read_self_maps();
/* Read brk after we've read the maps, which will malloc. */
brk = (uintptr_t)sbrk(0);
/* The first hole is before the first map entry. */
this_start = mmap_min_addr;
for (iter = maps; iter;
this_start = next_start, iter = g_slist_next(iter)) {
uintptr_t align_start, hole_size;
this_end = ((MapInfo *)iter->data)->start;
next_start = ((MapInfo *)iter->data)->end;
align_start = ROUND_UP(this_start, align);
/* Skip holes that are too small. */
if (align_start >= this_end) {
continue;
}
hole_size = this_end - align_start;
if (hole_size < guest_size) {
continue;
}
/* If this hole contains brk, give ourselves some room to grow. */
if (this_start <= brk && brk < this_end) {
hole_size -= guest_size;
if (sizeof(uintptr_t) == 8 && hole_size >= 1 * GiB) {
align_start += 1 * GiB;
} else if (hole_size >= 16 * MiB) {
align_start += 16 * MiB;
} else {
align_start = (this_end - guest_size) & -align;
if (align_start < this_start) {
continue;
}
}
}
/* Record the lowest successful match. */
if (ret < 0) {
ret = align_start - guest_loaddr;
}
/* If this hole contains the identity map, select it. */
if (align_start <= guest_loaddr &&
guest_loaddr + guest_size <= this_end) {
ret = 0;
}
/* If this hole ends above the identity map, stop looking. */
if (this_end >= guest_loaddr) {
break;
}
}
free_self_maps(maps);
return ret;
}
static void pgb_static(const char *image_name, abi_ulong orig_loaddr,
abi_ulong orig_hiaddr, long align)
{
uintptr_t loaddr = orig_loaddr;
uintptr_t hiaddr = orig_hiaddr;
uintptr_t addr;
if (hiaddr != orig_hiaddr) {
error_report("%s: requires virtual address space that the "
"host cannot provide (0x%" PRIx64 ")",
image_name, (uint64_t)orig_hiaddr);
exit(EXIT_FAILURE);
}
loaddr &= -align;
if (ARM_COMMPAGE) {
/*
* Extend the allocation to include the commpage.
* For a 64-bit host, this is just 4GiB; for a 32-bit host,
* the address arithmetic will wrap around, but the difference
* will produce the correct allocation size.
*/
if (sizeof(uintptr_t) == 8 || loaddr >= 0x80000000u) {
hiaddr = (uintptr_t)4 << 30;
} else {
loaddr = ARM_COMMPAGE & -align;
}
}
addr = pgb_find_hole(loaddr, hiaddr - loaddr, align);
if (addr == -1) {
/*
* If ARM_COMMPAGE, there *might* be a non-consecutive allocation
* that can satisfy both. But as the normal arm32 link base address
* is ~32k, and we extend down to include the commpage, making the
* overhead only ~96k, this is unlikely.
*/
error_report("%s: Unable to allocate %#zx bytes of "
"virtual address space", image_name,
(size_t)(hiaddr - loaddr));
exit(EXIT_FAILURE);
}
guest_base = addr;
}
static void pgb_dynamic(const char *image_name, long align)
{
/*
* The executable is dynamic and does not require a fixed address.
* All we need is a commpage that satisfies align.
* If we do not need a commpage, leave guest_base == 0.
*/
if (ARM_COMMPAGE) {
uintptr_t addr, commpage;
/* 64-bit hosts should have used reserved_va. */
assert(sizeof(uintptr_t) == 4);
/*
* By putting the commpage at the first hole, that puts guest_base
* just above that, and maximises the positive guest addresses.
*/
commpage = ARM_COMMPAGE & -align;
addr = pgb_find_hole(commpage, -commpage, align);
assert(addr != -1);
guest_base = addr;
}
}
static void pgb_reserved_va(const char *image_name, abi_ulong guest_loaddr,
abi_ulong guest_hiaddr, long align)
{
const int flags = MAP_ANONYMOUS | MAP_PRIVATE | MAP_NORESERVE;
void *addr, *test;
if (guest_hiaddr > reserved_va) {
error_report("%s: requires more than reserved virtual "
"address space (0x%" PRIx64 " > 0x%lx)",
image_name, (uint64_t)guest_hiaddr, reserved_va);
exit(EXIT_FAILURE);
}
/* Widen the "image" to the entire reserved address space. */
pgb_static(image_name, 0, reserved_va, align);
/* Reserve the memory on the host. */
assert(guest_base != 0);
test = g2h(0);
addr = mmap(test, reserved_va, PROT_NONE, flags, -1, 0);
if (addr == MAP_FAILED) {
error_report("Unable to reserve 0x%lx bytes of virtual address "
"space for use as guest address space (check your "
"virtual memory ulimit setting or reserve less "
"using -R option)", reserved_va);
exit(EXIT_FAILURE);
}
assert(addr == test);
}
void probe_guest_base(const char *image_name, abi_ulong guest_loaddr,
abi_ulong guest_hiaddr)
{ {
/* In order to use host shmat, we must be able to honor SHMLBA. */ /* In order to use host shmat, we must be able to honor SHMLBA. */
unsigned long align = MAX(SHMLBA, qemu_host_page_size); uintptr_t align = MAX(SHMLBA, qemu_host_page_size);
unsigned long current_start, aligned_start;
int flags;
assert(host_start || host_size); if (have_guest_base) {
pgb_have_guest_base(image_name, guest_loaddr, guest_hiaddr, align);
/* If just a starting address is given, then just verify that } else if (reserved_va) {
* address. */ pgb_reserved_va(image_name, guest_loaddr, guest_hiaddr, align);
if (host_start && !host_size) { } else if (guest_loaddr) {
#if defined(TARGET_ARM) && !defined(TARGET_AARCH64) pgb_static(image_name, guest_loaddr, guest_hiaddr, align);
if (init_guest_commpage(host_start, host_size) != 1) { } else {
return (unsigned long)-1; pgb_dynamic(image_name, align);
}
#endif
return host_start;
} }
/* Setup the initial flags and start address. */ /* Reserve and initialize the commpage. */
current_start = host_start & -align; if (!init_guest_commpage()) {
flags = MAP_ANONYMOUS | MAP_PRIVATE | MAP_NORESERVE; /*
if (fixed) { * With have_guest_base, the user has selected the address and
flags |= MAP_FIXED; * we are trying to work with that. Otherwise, we have selected
} * free space and init_guest_commpage must succeeded.
/* Otherwise, a non-zero size region of memory needs to be mapped
* and validated. */
#if defined(TARGET_ARM) && !defined(TARGET_AARCH64)
/* On 32-bit ARM, we need to map not just the usable memory, but
* also the commpage. Try to find a suitable place by allocating
* a big chunk for all of it. If host_start, then the naive
* strategy probably does good enough.
*/
if (!host_start) {
unsigned long guest_full_size, host_full_size, real_start;
guest_full_size =
(0xffff0f00 & qemu_host_page_mask) + qemu_host_page_size;
host_full_size = guest_full_size - guest_start;
real_start = (unsigned long)
mmap(NULL, host_full_size, PROT_NONE, flags, -1, 0);
if (real_start == (unsigned long)-1) {
if (host_size < host_full_size - qemu_host_page_size) {
/* We failed to map a continous segment, but we're
* allowed to have a gap between the usable memory and
* the commpage where other things can be mapped.
* This sparseness gives us more flexibility to find
* an address range.
*/
goto naive;
}
return (unsigned long)-1;
}
munmap((void *)real_start, host_full_size);
if (real_start & (align - 1)) {
/* The same thing again, but with extra
* so that we can shift around alignment.
*/
unsigned long real_size = host_full_size + qemu_host_page_size;
real_start = (unsigned long)
mmap(NULL, real_size, PROT_NONE, flags, -1, 0);
if (real_start == (unsigned long)-1) {
if (host_size < host_full_size - qemu_host_page_size) {
goto naive;
}
return (unsigned long)-1;
}
munmap((void *)real_start, real_size);
real_start = ROUND_UP(real_start, align);
}
current_start = real_start;
}
naive:
#endif
while (1) {
unsigned long real_start, real_size, aligned_size;
aligned_size = real_size = host_size;
/* Do not use mmap_find_vma here because that is limited to the
* guest address space. We are going to make the
* guest address space fit whatever we're given.
*/ */
real_start = (unsigned long) assert(have_guest_base);
mmap((void *)current_start, host_size, PROT_NONE, flags, -1, 0); pgb_fail_in_use(image_name);
if (real_start == (unsigned long)-1) {
return (unsigned long)-1;
}
/* Check to see if the address is valid. */
if (host_start && real_start != current_start) {
qemu_log_mask(CPU_LOG_PAGE, "invalid %lx && %lx != %lx\n",
host_start, real_start, current_start);
goto try_again;
}
/* Ensure the address is properly aligned. */
if (real_start & (align - 1)) {
/* Ideally, we adjust like
*
* pages: [ ][ ][ ][ ][ ]
* old: [ real ]
* [ aligned ]
* new: [ real ]
* [ aligned ]
*
* But if there is something else mapped right after it,
* then obviously it won't have room to grow, and the
* kernel will put the new larger real someplace else with
* unknown alignment (if we made it to here, then
* fixed=false). Which is why we grow real by a full page
* size, instead of by part of one; so that even if we get
* moved, we can still guarantee alignment. But this does
* mean that there is a padding of < 1 page both before
* and after the aligned range; the "after" could could
* cause problems for ARM emulation where it could butt in
* to where we need to put the commpage.
*/
munmap((void *)real_start, host_size);
real_size = aligned_size + align;
real_start = (unsigned long)
mmap((void *)real_start, real_size, PROT_NONE, flags, -1, 0);
if (real_start == (unsigned long)-1) {
return (unsigned long)-1;
}
aligned_start = ROUND_UP(real_start, align);
} else {
aligned_start = real_start;
}
#if defined(TARGET_ARM) && !defined(TARGET_AARCH64)
/* On 32-bit ARM, we need to also be able to map the commpage. */
int valid = init_guest_commpage(aligned_start - guest_start,
aligned_size + guest_start);
if (valid == -1) {
munmap((void *)real_start, real_size);
return (unsigned long)-1;
} else if (valid == 0) {
goto try_again;
}
#endif
/* If nothing has said `return -1` or `goto try_again` yet,
* then the address we have is good.
*/
break;
try_again:
/* That address didn't work. Unmap and try a different one.
* The address the host picked because is typically right at
* the top of the host address space and leaves the guest with
* no usable address space. Resort to a linear search. We
* already compensated for mmap_min_addr, so this should not
* happen often. Probably means we got unlucky and host
* address space randomization put a shared library somewhere
* inconvenient.
*
* This is probably a good strategy if host_start, but is
* probably a bad strategy if not, which means we got here
* because of trouble with ARM commpage setup.
*/
if (munmap((void *)real_start, real_size) != 0) {
error_report("%s: failed to unmap %lx:%lx (%s)", __func__,
real_start, real_size, strerror(errno));
abort();
}
current_start += align;
if (host_start == current_start) {
/* Theoretically possible if host doesn't have any suitably
* aligned areas. Normally the first mmap will fail.
*/
return (unsigned long)-1;
}
} }
qemu_log_mask(CPU_LOG_PAGE, "Reserved 0x%lx bytes of guest address space\n", host_size); assert(QEMU_IS_ALIGNED(guest_base, align));
qemu_log_mask(CPU_LOG_PAGE, "Locating guest address space "
return aligned_start; "@ 0x%" PRIx64 "\n", (uint64_t)guest_base);
} }
static void probe_guest_base(const char *image_name,
abi_ulong loaddr, abi_ulong hiaddr)
{
/* Probe for a suitable guest base address, if the user has not set
* it explicitly, and set guest_base appropriately.
* In case of error we will print a suitable message and exit.
*/
const char *errmsg;
if (!have_guest_base && !reserved_va) {
unsigned long host_start, real_start, host_size;
/* Round addresses to page boundaries. */
loaddr &= qemu_host_page_mask;
hiaddr = HOST_PAGE_ALIGN(hiaddr);
if (loaddr < mmap_min_addr) {
host_start = HOST_PAGE_ALIGN(mmap_min_addr);
} else {
host_start = loaddr;
if (host_start != loaddr) {
errmsg = "Address overflow loading ELF binary";
goto exit_errmsg;
}
}
host_size = hiaddr - loaddr;
/* Setup the initial guest memory space with ranges gleaned from
* the ELF image that is being loaded.
*/
real_start = init_guest_space(host_start, host_size, loaddr, false);
if (real_start == (unsigned long)-1) {
errmsg = "Unable to find space for application";
goto exit_errmsg;
}
guest_base = real_start - loaddr;
qemu_log_mask(CPU_LOG_PAGE, "Relocating guest address space from 0x"
TARGET_ABI_FMT_lx " to 0x%lx\n",
loaddr, real_start);
}
return;
exit_errmsg:
fprintf(stderr, "%s: %s\n", image_name, errmsg);
exit(-1);
}
/* Load an ELF image into the address space. /* Load an ELF image into the address space.
IMAGE_NAME is the filename of the image, to use in error messages. IMAGE_NAME is the filename of the image, to use in error messages.
@ -2399,6 +2390,12 @@ static void load_elf_image(const char *image_name, int image_fd,
* MMAP_MIN_ADDR or the QEMU application itself. * MMAP_MIN_ADDR or the QEMU application itself.
*/ */
probe_guest_base(image_name, loaddr, hiaddr); probe_guest_base(image_name, loaddr, hiaddr);
} else {
/*
* The binary is dynamic, but we still need to
* select guest_base. In this case we pass a size.
*/
probe_guest_base(image_name, 0, hiaddr - loaddr);
} }
} }

6
linux-user/flatload.c
View File

@ -441,6 +441,12 @@ static int load_flat_file(struct linux_binprm * bprm,
indx_len = MAX_SHARED_LIBS * sizeof(abi_ulong); indx_len = MAX_SHARED_LIBS * sizeof(abi_ulong);
indx_len = (indx_len + 15) & ~(abi_ulong)15; indx_len = (indx_len + 15) & ~(abi_ulong)15;
/*
* Alloate the address space.
*/
probe_guest_base(bprm->filename, 0,
text_len + data_len + extra + indx_len);
/* /*
* there are a couple of cases here, the separate code/data * there are a couple of cases here, the separate code/data
* case, and then the fully copied to RAM case which lumps * case, and then the fully copied to RAM case which lumps

23
linux-user/main.c
View File

@ -24,6 +24,7 @@
#include "qemu-version.h" #include "qemu-version.h"
#include <sys/syscall.h> #include <sys/syscall.h>
#include <sys/resource.h> #include <sys/resource.h>
#include <sys/shm.h>
#include "qapi/error.h" #include "qapi/error.h"
#include "qemu.h" #include "qemu.h"
@ -747,28 +748,6 @@ int main(int argc, char **argv, char **envp)
target_environ = envlist_to_environ(envlist, NULL); target_environ = envlist_to_environ(envlist, NULL);
envlist_free(envlist); envlist_free(envlist);
/*
* Now that page sizes are configured in tcg_exec_init() we can do
* proper page alignment for guest_base.
*/
guest_base = HOST_PAGE_ALIGN(guest_base);
if (reserved_va || have_guest_base) {
guest_base = init_guest_space(guest_base, reserved_va, 0,
have_guest_base);
if (guest_base == (unsigned long)-1) {
fprintf(stderr, "Unable to reserve 0x%lx bytes of virtual address "
"space for use as guest address space (check your virtual "
"memory ulimit setting or reserve less using -R option)\n",
reserved_va);
exit(EXIT_FAILURE);
}
if (reserved_va) {
mmap_next_start = reserved_va;
}
}
/* /*
* Read in mmap_min_addr kernel parameter. This value is used * Read in mmap_min_addr kernel parameter. This value is used
* When loading the ELF image to determine whether guest_base * When loading the ELF image to determine whether guest_base

31
linux-user/qemu.h
View File

@ -219,18 +219,27 @@ void init_qemu_uname_release(void);
void fork_start(void); void fork_start(void);
void fork_end(int child); void fork_end(int child);
/* Creates the initial guest address space in the host memory space using /**
* the given host start address hint and size. The guest_start parameter * probe_guest_base:
* specifies the start address of the guest space. guest_base will be the * @image_name: the executable being loaded
* difference between the host start address computed by this function and * @loaddr: the lowest fixed address in the executable
* guest_start. If fixed is specified, then the mapped address space must * @hiaddr: the highest fixed address in the executable
* start at host_start. The real start address of the mapped memory space is *
* returned or -1 if there was an error. * Creates the initial guest address space in the host memory space.
*
* If @loaddr == 0, then no address in the executable is fixed,
* i.e. it is fully relocatable. In that case @hiaddr is the size
* of the executable.
*
* This function will not return if a valid value for guest_base
* cannot be chosen. On return, the executable loader can expect
*
* target_mmap(loaddr, hiaddr - loaddr, ...)
*
* to succeed.
*/ */
unsigned long init_guest_space(unsigned long host_start, void probe_guest_base(const char *image_name,
unsigned long host_size, abi_ulong loaddr, abi_ulong hiaddr);
unsigned long guest_start,
bool fixed);
#include "qemu/log.h" #include "qemu/log.h"