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linux-user: Allow gdbstub to ignore page protection

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gdbserver ignores page protection by virtue of using /proc/$pid/mem.
Teach qemu gdbstub to do this too. This will not work if /proc is not
mounted; accept this limitation.

One alternative is to temporarily grant the missing PROT_* bit, but
this is inherently racy. Another alternative is self-debugging with
ptrace(POKE), which will break if QEMU itself is being debugged - a
much more severe limitation.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Message-Id: <20240129093410.3151-2-iii@linux.ibm.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
This commit is contained in:
Ilya Leoshkevich 2024-01-29 10:32:14 +01:00 committed by Richard Henderson
parent fd3f7d24d4
commit 87ab270429
1 changed files with 63 additions and 15 deletions
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78
cpu-target.c
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@ -382,6 +382,9 @@ int cpu_memory_rw_debug(CPUState *cpu, vaddr addr,
vaddr l, page;
void * p;
uint8_t *buf = ptr;
ssize_t written;
int ret = -1;
int fd = -1;
while (len > 0) {
page = addr & TARGET_PAGE_MASK;
@ -389,30 +392,75 @@ int cpu_memory_rw_debug(CPUState *cpu, vaddr addr,
if (l > len)
l = len;
flags = page_get_flags(page);
if (!(flags & PAGE_VALID))
return -1;
if (!(flags & PAGE_VALID)) {
goto out_close;
}
if (is_write) {
if (!(flags & PAGE_WRITE))
return -1;
if (flags & PAGE_WRITE) {
/* XXX: this code should not depend on lock_user */
p = lock_user(VERIFY_WRITE, addr, l, 0);
if (!p) {
goto out_close;
}
memcpy(p, buf, l);
unlock_user(p, addr, l);
} else {
/* Bypass the host page protection using ptrace. */
if (fd == -1) {
fd = open("/proc/self/mem", O_WRONLY);
if (fd == -1) {
goto out;
}
}
/*
* If there is a TranslationBlock and we weren't bypassing the
* host page protection, the memcpy() above would SEGV,
* ultimately leading to page_unprotect(). So invalidate the
* translations manually. Both invalidation and pwrite() must
* be under mmap_lock() in order to prevent the creation of
* another TranslationBlock in between.
*/
mmap_lock();
tb_invalidate_phys_range(addr, addr + l - 1);
written = pwrite(fd, buf, l,
(off_t)(uintptr_t)g2h_untagged(addr));
mmap_unlock();
if (written != l) {
goto out_close;
}
}
} else if (flags & PAGE_READ) {
/* XXX: this code should not depend on lock_user */
if (!(p = lock_user(VERIFY_WRITE, addr, l, 0)))
return -1;
memcpy(p, buf, l);
unlock_user(p, addr, l);
} else {
if (!(flags & PAGE_READ))
return -1;
/* XXX: this code should not depend on lock_user */
if (!(p = lock_user(VERIFY_READ, addr, l, 1)))
return -1;
p = lock_user(VERIFY_READ, addr, l, 1);
if (!p) {
goto out_close;
}
memcpy(buf, p, l);
unlock_user(p, addr, 0);
} else {
/* Bypass the host page protection using ptrace. */
if (fd == -1) {
fd = open("/proc/self/mem", O_RDONLY);
if (fd == -1) {
goto out;
}
}
if (pread(fd, buf, l,
(off_t)(uintptr_t)g2h_untagged(addr)) != l) {
goto out_close;
}
}
len -= l;
buf += l;
addr += l;
}
return 0;
ret = 0;
out_close:
if (fd != -1) {
close(fd);
}
out:
return ret;
}
#endif