From: Johannes Berg <johannes.b...@intel.com>
Since __attribute__((naked)) cannot be used with functions
containing C statements, just generate the few instructions
it needs in assembly directly.
While at it, fix the stack usage ("1 + 2*x - 1" is odd) and
document what it must do, and why it must adjust the stack.
Fixes: 8508a5e0e9db ("um: Fix misaligned stack in stub_exe")
Link:
https://lore.kernel.org/linux-um/cabvgosnth-uoofmp5hwmxjx_f1osmnvdhgkrkm4uz6dfm2l...@mail.gmail.com/
Signed-off-by: Johannes Berg <johannes.b...@intel.com>
---
arch/um/kernel/skas/stub_exe.c | 18 +++++++++++-------
arch/x86/um/shared/sysdep/stub_32.h | 8 ++++++++
arch/x86/um/shared/sysdep/stub_64.h | 8 ++++++++
3 files changed, 27 insertions(+), 7 deletions(-)
diff --git a/arch/um/kernel/skas/stub_exe.c b/arch/um/kernel/skas/stub_exe.c
index 722ce6267476..ff5471986c52 100644
--- a/arch/um/kernel/skas/stub_exe.c
+++ b/arch/um/kernel/skas/stub_exe.c
@@ -81,11 +81,15 @@ noinline static void real_init(void)
__attribute__((naked)) void _start(void)
{
- char *alloc;
-
- /* Make enough space for the stub (including space for alignment) */
- alloc = __builtin_alloca((1 + 2 * STUB_DATA_PAGES - 1) *
UM_KERN_PAGE_SIZE);
- asm volatile("" : "+r,m"(alloc) : : "memory");
-
- real_init();
+ /*
+ * Since the stack after exec() starts at the top-most address,
+ * but that's exactly where we also want to map the stub data
+ * and code, this must:
+ * - push the stack by 1 code and STUB_DATA_PAGES data pages
+ * - call real_init()
+ * This way, real_init() can use the stack normally, while the
+ * original stack further down (higher address) is no longer
+ * accessible after the mmap() calls above.
+ */
+ stub_start(real_init);
}
diff --git a/arch/x86/um/shared/sysdep/stub_32.h
b/arch/x86/um/shared/sysdep/stub_32.h
index 631a18d0ff44..390988132c0a 100644
--- a/arch/x86/um/shared/sysdep/stub_32.h
+++ b/arch/x86/um/shared/sysdep/stub_32.h
@@ -123,4 +123,12 @@ static __always_inline void *get_stub_data(void)
return (void *)ret;
}
+
+#define stub_start(fn) \
+ asm volatile ( \
+ "subl %0,%%esp ;" \
+ "movl %1, %%eax ; " \
+ "call *%%eax ;" \
+ :: "i" ((1 + STUB_DATA_PAGES) * UM_KERN_PAGE_SIZE), \
+ "i" (&fn))
#endif
diff --git a/arch/x86/um/shared/sysdep/stub_64.h
b/arch/x86/um/shared/sysdep/stub_64.h
index 17153dfd780a..294affbec742 100644
--- a/arch/x86/um/shared/sysdep/stub_64.h
+++ b/arch/x86/um/shared/sysdep/stub_64.h
@@ -126,4 +126,12 @@ static __always_inline void *get_stub_data(void)
return (void *)ret;
}
+
+#define stub_start(fn) \
+ asm volatile ( \
+ "subq %0,%%rsp ;" \
+ "movq %1,%%rax ;" \
+ "call *%%rax ;" \
+ :: "i" ((1 + STUB_DATA_PAGES) * UM_KERN_PAGE_SIZE), \
+ "i" (&fn))
#endif
--
2.47.0
