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linux-user: Split out mmap_h_lt_g

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Work much harder to get alignment and mapping beyond the end
of the file correct.  Both of which are excercised by our
test-mmap for alpha (8k pages) on any 4k page host.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Acked-by: Helge Deller <deller@gmx.de>
Message-Id: <20240102015808.132373-23-richard.henderson@linaro.org>
This commit is contained in:
Richard Henderson 2024-01-02 12:57:57 +11:00
parent 68098de90e
commit 8080b2f804
1 changed files with 153 additions and 31 deletions
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184
linux-user/mmap.c
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@ -569,6 +569,156 @@ static abi_long mmap_h_eq_g(abi_ulong start, abi_ulong len,
return mmap_end(start, last, start, last, flags, page_flags);
}
/*
* Special case host page size < target page size.
*
* The two special cases are increased guest alignment, and mapping
* past the end of a file.
*
* When mapping files into a memory area larger than the file,
* accesses to pages beyond the file size will cause a SIGBUS.
*
* For example, if mmaping a file of 100 bytes on a host with 4K
* pages emulating a target with 8K pages, the target expects to
* be able to access the first 8K. But the host will trap us on
* any access beyond 4K.
*
* When emulating a target with a larger page-size than the hosts,
* we may need to truncate file maps at EOF and add extra anonymous
* pages up to the targets page boundary.
*
* This workaround only works for files that do not change.
* If the file is later extended (e.g. ftruncate), the SIGBUS
* vanishes and the proper behaviour is that changes within the
* anon page should be reflected in the file.
*
* However, this case is rather common with executable images,
* so the workaround is important for even trivial tests, whereas
* the mmap of of a file being extended is less common.
*/
static abi_long mmap_h_lt_g(abi_ulong start, abi_ulong len, int host_prot,
int mmap_flags, int page_flags, int fd,
off_t offset, int host_page_size)
{
void *p, *want_p = g2h_untagged(start);
off_t fileend_adj = 0;
int flags = mmap_flags;
abi_ulong last, pass_last;
if (!(flags & MAP_ANONYMOUS)) {
struct stat sb;
if (fstat(fd, &sb) == -1) {
return -1;
}
if (offset >= sb.st_size) {
/*
* The entire map is beyond the end of the file.
* Transform it to an anonymous mapping.
*/
flags |= MAP_ANONYMOUS;
fd = -1;
offset = 0;
} else if (offset + len > sb.st_size) {
/*
* A portion of the map is beyond the end of the file.
* Truncate the file portion of the allocation.
*/
fileend_adj = offset + len - sb.st_size;
}
}
if (flags & (MAP_FIXED | MAP_FIXED_NOREPLACE)) {
if (fileend_adj) {
p = mmap(want_p, len, host_prot, flags | MAP_ANONYMOUS, -1, 0);
} else {
p = mmap(want_p, len, host_prot, flags, fd, offset);
}
if (p != want_p) {
if (p != MAP_FAILED) {
/* Host does not support MAP_FIXED_NOREPLACE: emulate. */
do_munmap(p, len);
errno = EEXIST;
}
return -1;
}
if (fileend_adj) {
void *t = mmap(p, len - fileend_adj, host_prot,
(flags & ~MAP_FIXED_NOREPLACE) | MAP_FIXED,
fd, offset);
if (t == MAP_FAILED) {
int save_errno = errno;
/*
* We failed a map over the top of the successful anonymous
* mapping above. The only failure mode is running out of VMAs,
* and there's nothing that we can do to detect that earlier.
* If we have replaced an existing mapping with MAP_FIXED,
* then we cannot properly recover. It's a coin toss whether
* it would be better to exit or continue here.
*/
if (!(flags & MAP_FIXED_NOREPLACE) &&
!page_check_range_empty(start, start + len - 1)) {
qemu_log("QEMU target_mmap late failure: %s",
strerror(save_errno));
}
do_munmap(want_p, len);
errno = save_errno;
return -1;
}
}
} else {
size_t host_len, part_len;
/*
* Take care to align the host memory. Perform a larger anonymous
* allocation and extract the aligned portion. Remap the file on
* top of that.
*/
host_len = len + TARGET_PAGE_SIZE - host_page_size;
p = mmap(want_p, host_len, host_prot, flags | MAP_ANONYMOUS, -1, 0);
if (p == MAP_FAILED) {
return -1;
}
part_len = (uintptr_t)p & (TARGET_PAGE_SIZE - 1);
if (part_len) {
part_len = TARGET_PAGE_SIZE - part_len;
do_munmap(p, part_len);
p += part_len;
host_len -= part_len;
}
if (len < host_len) {
do_munmap(p + len, host_len - len);
}
if (!(flags & MAP_ANONYMOUS)) {
void *t = mmap(p, len - fileend_adj, host_prot,
flags | MAP_FIXED, fd, offset);
if (t == MAP_FAILED) {
int save_errno = errno;
do_munmap(p, len);
errno = save_errno;
return -1;
}
}
start = h2g(p);
}
last = start + len - 1;
if (fileend_adj) {
pass_last = ROUND_UP(last - fileend_adj, host_page_size) - 1;
} else {
pass_last = last;
}
return mmap_end(start, last, start, pass_last, mmap_flags, page_flags);
}
static abi_long target_mmap__locked(abi_ulong start, abi_ulong len,
int target_prot, int flags, int page_flags,
int fd, off_t offset)
@ -613,37 +763,9 @@ static abi_long target_mmap__locked(abi_ulong start, abi_ulong len,
if (host_page_size == TARGET_PAGE_SIZE) {
return mmap_h_eq_g(start, len, host_prot, flags,
page_flags, fd, offset);
}
/*
* When mapping files into a memory area larger than the file, accesses
* to pages beyond the file size will cause a SIGBUS.
*
* For example, if mmaping a file of 100 bytes on a host with 4K pages
* emulating a target with 8K pages, the target expects to be able to
* access the first 8K. But the host will trap us on any access beyond
* 4K.
*
* When emulating a target with a larger page-size than the hosts, we
* may need to truncate file maps at EOF and add extra anonymous pages
* up to the targets page boundary.
*/
if (host_page_size < TARGET_PAGE_SIZE && !(flags & MAP_ANONYMOUS)) {
struct stat sb;
if (fstat(fd, &sb) == -1) {
return -1;
}
/* Are we trying to create a map beyond EOF?. */
if (offset + len > sb.st_size) {
/*
* If so, truncate the file map at eof aligned with
* the hosts real pagesize. Additional anonymous maps
* will be created beyond EOF.
*/
len = ROUND_UP(sb.st_size - offset, host_page_size);
}
} else if (host_page_size < TARGET_PAGE_SIZE) {
return mmap_h_lt_g(start, len, host_prot, flags,
page_flags, fd, offset, host_page_size);
}
if (!(flags & (MAP_FIXED | MAP_FIXED_NOREPLACE))) {