Commit e8f3010f7326 ("arm64/efi: isolate EFI stub from the kernel
proper") isolated the EFI stub code from the kernel proper by prefixing
all of its symbols with __efistub_, and selectively allowing access to
core kernel symbols from the stub by emitting __efistub_ aliases for
functions and variables that the stub can access legally.
As an unintended side effect, these aliases are emitted into the
kallsyms symbol table, which means they may turn up in backtraces,
e.g.,
...
PC is at __efistub_memset+0x108/0x200
LR is at fixup_init+0x3c/0x48
...
[<ffffff8008328608>] __efistub_memset+0x108/0x200
[<ffffff8008094dcc>] free_initmem+0x2c/0x40
[<ffffff8008645198>] kernel_init+0x20/0xe0
[<ffffff8008085cd0>] ret_from_fork+0x10/0x40
The backtrace in question has nothing to do with the EFI stub, but
simply returns one of the several aliases of memset() that have been
recorded in the kallsyms table. This is undesirable, since it may
suggest to people who are not aware of this that the issue they are
seeing is somehow EFI related.
So hide the __efistub_ aliases from kallsyms, by emitting them as
absolute linker symbols explicitly. The distinction between those
and section relative symbols is completely irrelevant to these
definitions, and to the final link we are performing when these
definitions are being taken into account (the distinction is only
relevant to symbols defined inside a section definition when performing
a partial link), and so the resulting values are identical to the
original ones. Since absolute symbols are ignored by kallsyms, this
will result in these values to be omitted from its symbol table.
After this patch, the backtrace generated from the same address looks
like this:
...
PC is at __memset+0x108/0x200
LR is at fixup_init+0x3c/0x48
...
[<ffffff8008328608>] __memset+0x108/0x200
[<ffffff8008094dcc>] free_initmem+0x2c/0x40
[<ffffff8008645198>] kernel_init+0x20/0xe0
[<ffffff8008085cd0>] ret_from_fork+0x10/0x40
Signed-off-by: Ard Biesheuvel <redacted>
---
arch/arm64/kernel/image.h | 40 +++++++++++++++++++++++++---------------
1 file changed, 25 insertions(+), 15 deletions(-)
On Fri, Jan 15, 2016 at 01:28:57PM +0100, Ard Biesheuvel wrote:
quoted hunk
Commit e8f3010f7326 ("arm64/efi: isolate EFI stub from the kernel
proper") isolated the EFI stub code from the kernel proper by prefixing
all of its symbols with __efistub_, and selectively allowing access to
core kernel symbols from the stub by emitting __efistub_ aliases for
functions and variables that the stub can access legally.
As an unintended side effect, these aliases are emitted into the
kallsyms symbol table, which means they may turn up in backtraces,
e.g.,
...
PC is at __efistub_memset+0x108/0x200
LR is at fixup_init+0x3c/0x48
...
[<ffffff8008328608>] __efistub_memset+0x108/0x200
[<ffffff8008094dcc>] free_initmem+0x2c/0x40
[<ffffff8008645198>] kernel_init+0x20/0xe0
[<ffffff8008085cd0>] ret_from_fork+0x10/0x40
The backtrace in question has nothing to do with the EFI stub, but
simply returns one of the several aliases of memset() that have been
recorded in the kallsyms table. This is undesirable, since it may
suggest to people who are not aware of this that the issue they are
seeing is somehow EFI related.
So hide the __efistub_ aliases from kallsyms, by emitting them as
absolute linker symbols explicitly. The distinction between those
and section relative symbols is completely irrelevant to these
definitions, and to the final link we are performing when these
definitions are being taken into account (the distinction is only
relevant to symbols defined inside a section definition when performing
a partial link), and so the resulting values are identical to the
original ones. Since absolute symbols are ignored by kallsyms, this
will result in these values to be omitted from its symbol table.
After this patch, the backtrace generated from the same address looks
like this:
...
PC is at __memset+0x108/0x200
LR is at fixup_init+0x3c/0x48
...
[<ffffff8008328608>] __memset+0x108/0x200
[<ffffff8008094dcc>] free_initmem+0x2c/0x40
[<ffffff8008645198>] kernel_init+0x20/0xe0
[<ffffff8008085cd0>] ret_from_fork+0x10/0x40
Signed-off-by: Ard Biesheuvel <redacted>
---
arch/arm64/kernel/image.h | 40 +++++++++++++++++++++++++---------------
1 file changed, 25 insertions(+), 15 deletions(-)
From looking at the GNU LD manual recently, I discovered
PROVIDE_HIDDEN(sym), which sounds closer to what we want semantically,
even if ABSOLUTE(sym) acheives the same thing.
If that works, that might be preferable.
Either way this looks sensible to me, so:
Acked-by: Mark Rutland <mark.rutland@arm.com>
Mark.
quoted hunk
++/* * The EFI stub has its own symbol namespace prefixed by __efistub_, to * isolate it from the kernel proper. The following symbols are legally * accessed by the stub, so provide some aliases to make them accessible.
On 15 January 2016 at 13:41, Mark Rutland [off-list ref] wrote:
On Fri, Jan 15, 2016 at 01:28:57PM +0100, Ard Biesheuvel wrote:
quoted
Commit e8f3010f7326 ("arm64/efi: isolate EFI stub from the kernel
proper") isolated the EFI stub code from the kernel proper by prefixing
all of its symbols with __efistub_, and selectively allowing access to
core kernel symbols from the stub by emitting __efistub_ aliases for
functions and variables that the stub can access legally.
As an unintended side effect, these aliases are emitted into the
kallsyms symbol table, which means they may turn up in backtraces,
e.g.,
...
PC is at __efistub_memset+0x108/0x200
LR is at fixup_init+0x3c/0x48
...
[<ffffff8008328608>] __efistub_memset+0x108/0x200
[<ffffff8008094dcc>] free_initmem+0x2c/0x40
[<ffffff8008645198>] kernel_init+0x20/0xe0
[<ffffff8008085cd0>] ret_from_fork+0x10/0x40
The backtrace in question has nothing to do with the EFI stub, but
simply returns one of the several aliases of memset() that have been
recorded in the kallsyms table. This is undesirable, since it may
suggest to people who are not aware of this that the issue they are
seeing is somehow EFI related.
So hide the __efistub_ aliases from kallsyms, by emitting them as
absolute linker symbols explicitly. The distinction between those
and section relative symbols is completely irrelevant to these
definitions, and to the final link we are performing when these
definitions are being taken into account (the distinction is only
relevant to symbols defined inside a section definition when performing
a partial link), and so the resulting values are identical to the
original ones. Since absolute symbols are ignored by kallsyms, this
will result in these values to be omitted from its symbol table.
After this patch, the backtrace generated from the same address looks
like this:
...
PC is at __memset+0x108/0x200
LR is at fixup_init+0x3c/0x48
...
[<ffffff8008328608>] __memset+0x108/0x200
[<ffffff8008094dcc>] free_initmem+0x2c/0x40
[<ffffff8008645198>] kernel_init+0x20/0xe0
[<ffffff8008085cd0>] ret_from_fork+0x10/0x40
Signed-off-by: Ard Biesheuvel <redacted>
---
arch/arm64/kernel/image.h | 40 +++++++++++++++++++++++++---------------
1 file changed, 25 insertions(+), 15 deletions(-)
From looking at the GNU LD manual recently, I discovered
PROVIDE_HIDDEN(sym), which sounds closer to what we want semantically,
even if ABSOLUTE(sym) acheives the same thing.
If that works, that might be preferable.
No, it doesn't. PROVIDE_HIDDEN() turns it into a local symbol, but
preserves the type, i.e.,
Before:
ffffff8008080000 T __efistub__text
With this patch
ffffff8008080000 A __efistub__text
With PROVIDE_HIDDEN
ffffff8008080000 t __efistub__text
I am not crazy about this change myself, but at least it keeps the
ugliness confined to image.h in a well annotated manner, rather than
in every backtrace that involves any of these functions.
Either way this looks sensible to me, so:
Acked-by: Mark Rutland <mark.rutland@arm.com>
Thanks,
Ard.
quoted
++/* * The EFI stub has its own symbol namespace prefixed by __efistub_, to * isolate it from the kernel proper. The following symbols are legally * accessed by the stub, so provide some aliases to make them accessible.