On 8/21/2026 10:06 PM, [email protected] wrote:
> In preparation for unwinding user space stacks with .eh_frame, add
> basic unwind user eh_frame infrastructure and support for reading the
> .eh_frame_hdr section.
>> diff --git a/kernel/unwind/eh_frame.c b/kernel/unwind/eh_frame.c
#include <linux/sched/task_stack.h>
#include <linux/unwind_user.h>
>> @@ -0,0 +1,366 @@
>
> [ ... ]
>
>> +static __always_inline int encoded_pointer_size(u8 encoding)
>> +{
>> + u8 format = DW_EH_PE_format(encoding);
>> +
>> + switch (format) {
>> + case DW_EH_PE_absptr:
if (IS_ENABLED(CONFIG_COMPAT))
return unwind_user_word_size(task_pt_regs(current));
>> + return sizeof(unsigned long);
>
> [Severity: High]
> Does hardcoding sizeof(unsigned long) here cause issues for 32-bit compat
> tasks?
The word size from struct unwind_user_state field word_size cannot be used
because it needs to be known at .eh_frame[_hdr] section registration time:
eh_frame_add_section() -> eh_frame_read_header() -> read_encoded_pointer()
Would it be ok to determine the word size each time the special case
DW_EH_PE_absptr is handled in encoded_pointer_size() and
read_encoded_pointer()? See also below.
Also I need to do some testing whether those actually occur in .eh_frame.
If not it would be preferable to drop DW_EH_PE_absptr support.
>
> If a 64-bit kernel is profiling a 32-bit application, DW_EH_PE_absptr is
> expected to match the application's native 4-byte pointer size. Using
> sizeof(unsigned long) means the kernel will read 8 bytes, which could
> consume too much data and misalign the current pointer for the remainder
> of the unwinding process.
>
>> + case DW_EH_PE_udata2:
>> + case DW_EH_PE_sdata2:
>> + return 2;
>
> [ ... ]
>
>> +static __always_inline int read_encoded_pointer(struct eh_frame_section
>> *sec,
>> + unsigned long *addr,
>> + unsigned long end,
>> + u8 encoding,
>> + unsigned long *value)
>> +{
>
> [ ... ]
>
>> + /* Read value based on format */
>> + switch (format) {
>> + case DW_EH_PE_absptr: {
>> + unsigned long tmp;
>> + UNSAFE_GET_USER_INC(tmp, cur, end, Efault);
>> + result += tmp;
Replace above with:
if (IS_ENABLED(CONFIG_COMPAT) &&
unwind_user_word_size(task_pt_regs(current)) ==
sizeof(int)) {
unsigned int tmp32;
UNSAFE_GET_USER_INC(tmp32, cur, end, Efault);
result += tmp32;
} else {
unsigned long tmp;
UNSAFE_GET_USER_INC(tmp, cur, end, Efault);
result += tmp;
}
>> + break;
>> + }
>
> [Severity: High]
> Will this also read 8 bytes instead of 4 for a 32-bit task?
>
> Similar to encoded_pointer_size(), this reads a full unsigned long for an
> absolute pointer. If this is a 64-bit kernel unwinding a 32-bit compat
> application, it seems this might read past the intended data boundaries
> and corrupt the address parsing.
>
>> + case DW_EH_PE_uleb128: {
>> + unsigned long tmp;
>
> [ ... ]
>
>> +static int eh_frame_read_header(struct eh_frame_section *sec)
>> +{
>> + struct mm_struct *mm = current->mm;
>> + void __user *eh_frame_hdr = (void __user *)sec->eh_frame_hdr_start;
size_t size = sec->eh_frame_hdr_end - sec->eh_frame_hdr_start;
>> + unsigned long cur = sec->eh_frame_hdr_start, end =
>> sec->eh_frame_hdr_end;
Move cur and end into scoped user access block below.
>
> [ ... ]
>
>> + scoped_user_read_access_size(eh_frame_hdr, end -
>> sec->eh_frame_hdr_start,
>> + Efault) {
scoped_user_read_access_size(eh_frame_hdr, size, Efault) {
unsigned long cur = (unsigned long)eh_frame_hdr;
unsigned long end = (unsigned long)eh_frame_hdr + size;
>> + /* Read version */
>> + UNSAFE_GET_USER_INC(version, cur, end, Efault);
Then whenever cur or end are expected to be unmasked pointers they need
to be converted back:
- table_start = cur;
+ table_start = sec->eh_frame_hdr_start + (cur - (unsigned
long)eh_frame_hdr);
...
- if (fde_count > (end - table_start) / entry_size)
+ if (fde_count > (sec->eh_frame_hdr_end - table_start) /
entry_size)
return -EINVAL;
>
> [Severity: High]
> Does this bypass the speculative execution pointer masking provided by
> scoped_user_read_access_size()?
True. I was not aware of that. While above seems to resolve this at a
first glance there are some implications in read_encoded_pointer():
DW_EH_PE_datarel (relative to .eh_frame_hdr) and DW_EH_PE_textrel
(relative to .text) return an unmasked pointer, which is ok, as the
callers expect it to be an unmasked value.
DW_EH_PE_pcrel (relative to the field itself) also returns an unmasked
pointer and now requires the unmasked cursor pointer to be provided as
input as well.
>
> The block masks the eh_frame_hdr pointer, but inside the block,
> UNSAFE_GET_USER_INC() reads from the unmasked unsigned long cur variable:
>
> #define UNSAFE_GET_USER_INC(to, from, end, label)
> ...
> unsafe_get_user(__to, (typeof(to) __user *)from, label);
>
> Since cur is passed to unsafe_get_user() without being masked, it appears
> this defeats the Spectre V1 mitigation for architectures relying on
> can_do_masked_user_access().
>
>> + if (version != 1)
>> + return -EINVAL;
>> +int eh_frame_add_section(unsigned long eh_frame_hdr_start,
>> + unsigned long eh_frame_hdr_end,
>> + unsigned long text_start,
>> + unsigned long text_end)
>> +{
>> + struct mm_struct *mm = current->mm;
>> + struct eh_frame_section *sec;
>> + int ret;
>
> [ ... ]
>
>> + scoped_guard(mmap_read_lock, mm) {
>> + struct vm_area_struct *eh_frame_hdr_vma, *text_vma;
>> +
>> + eh_frame_hdr_vma = vma_lookup(mm, eh_frame_hdr_start);
>
> [ ... ]
>
>> + ret = eh_frame_read_header(sec);
>> + if (ret)
>> + goto err_free;
>> +
>> + /* TODO nowhere to store it yet - just free it and return an error */
>> + ret = -ENOSYS;
>> +
>> +err_free:
>> + free_section(sec);
>> + return ret;
>> +}
>
> [Severity: Low]
> This isn't a bug, but should this function avoid mixing scoped_guard() and
> goto-based error handling?
>
> The kernel cleanup guidelines recommend either fully converting to
> scope-based cleanup (for example, using __free() for the sec allocation)
> or strictly using goto-based cleanups without scoped_guard(). Mixing them
> in the same function can create confusing ownership semantics.
Other than converting the code to use mmap_read_lock(mm) and
mmap_read_unlock(mm) I don't see how the section cleanup logic
could be converted to scope-based cleanup, as it must not be
freed if it gets inserted into the mm->eh_frame_mt (in a
subsequent patch). Sashiko probably misses this as in this
incomplete state it looks as if it could be converted, as the
section is always freed.
Thanks and regards,
Jens
--
Jens Remus
Linux on Z Development (D3303)
[email protected] / [email protected]
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