From: "Eric W. Biederman" <[email protected]>
Update the static assertions about siginfo_t to also describe
it's alignment and size.
While investigating if it was possible to add a 64bit field into
siginfo_t[1] it became apparent that the alignment of siginfo_t
is as much a part of the ABI as the size of the structure.
If the alignment changes siginfo_t when embedded in another structure
can move to a different offset. Which is not acceptable from an ABI
structure.
So document that fact and add static assertions to notify developers
if they change change the alignment by accident.
[1] https://lkml.kernel.org/r/[email protected]
Signed-off-by: "Eric W. Biederman" <[email protected]>
---
arch/arm/kernel/signal.c | 2 ++
arch/arm64/kernel/signal.c | 2 ++
arch/arm64/kernel/signal32.c | 2 ++
arch/sparc/kernel/signal32.c | 2 ++
arch/sparc/kernel/signal_64.c | 2 ++
arch/x86/kernel/signal_compat.c | 6 ++++++
include/uapi/asm-generic/siginfo.h | 5 +++++
7 files changed, 21 insertions(+)
diff --git a/arch/arm/kernel/signal.c b/arch/arm/kernel/signal.c
index 2dac5d2c5cf6..643bcb0f091b 100644
--- a/arch/arm/kernel/signal.c
+++ b/arch/arm/kernel/signal.c
@@ -737,6 +737,8 @@ static_assert(NSIGBUS == 5);
static_assert(NSIGTRAP == 6);
static_assert(NSIGCHLD == 6);
static_assert(NSIGSYS == 2);
+static_assert(sizeof(siginfo_t) == 128);
+static_assert(__alignof__(siginfo_t) == 4);
static_assert(offsetof(siginfo_t, si_signo) == 0x00);
static_assert(offsetof(siginfo_t, si_errno) == 0x04);
static_assert(offsetof(siginfo_t, si_code) == 0x08);
diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
index af8bd2af1298..ad4bd27fc044 100644
--- a/arch/arm64/kernel/signal.c
+++ b/arch/arm64/kernel/signal.c
@@ -985,6 +985,8 @@ static_assert(NSIGBUS == 5);
static_assert(NSIGTRAP == 6);
static_assert(NSIGCHLD == 6);
static_assert(NSIGSYS == 2);
+static_assert(sizeof(siginfo_t) == 128);
+static_assert(__alignof__(siginfo_t) == 8);
static_assert(offsetof(siginfo_t, si_signo) == 0x00);
static_assert(offsetof(siginfo_t, si_errno) == 0x04);
static_assert(offsetof(siginfo_t, si_code) == 0x08);
diff --git a/arch/arm64/kernel/signal32.c b/arch/arm64/kernel/signal32.c
index b6afb646515f..ee6c7484e130 100644
--- a/arch/arm64/kernel/signal32.c
+++ b/arch/arm64/kernel/signal32.c
@@ -469,6 +469,8 @@ static_assert(NSIGBUS == 5);
static_assert(NSIGTRAP == 6);
static_assert(NSIGCHLD == 6);
static_assert(NSIGSYS == 2);
+static_assert(sizeof(compat_siginfo_t) == 128);
+static_assert(__alignof__(compat_siginfo_t) == 4);
static_assert(offsetof(compat_siginfo_t, si_signo) == 0x00);
static_assert(offsetof(compat_siginfo_t, si_errno) == 0x04);
static_assert(offsetof(compat_siginfo_t, si_code) == 0x08);
diff --git a/arch/sparc/kernel/signal32.c b/arch/sparc/kernel/signal32.c
index 778ed5c26d4a..32b977f253e3 100644
--- a/arch/sparc/kernel/signal32.c
+++ b/arch/sparc/kernel/signal32.c
@@ -757,6 +757,8 @@ static_assert(NSIGBUS == 5);
static_assert(NSIGTRAP == 6);
static_assert(NSIGCHLD == 6);
static_assert(NSIGSYS == 2);
+static_assert(sizeof(compat_siginfo_t) == 128);
+static_assert(__alignof__(compat_siginfo_t) == 4);
static_assert(offsetof(compat_siginfo_t, si_signo) == 0x00);
static_assert(offsetof(compat_siginfo_t, si_errno) == 0x04);
static_assert(offsetof(compat_siginfo_t, si_code) == 0x08);
diff --git a/arch/sparc/kernel/signal_64.c b/arch/sparc/kernel/signal_64.c
index c9bbf5f29078..e9dda9db156c 100644
--- a/arch/sparc/kernel/signal_64.c
+++ b/arch/sparc/kernel/signal_64.c
@@ -567,6 +567,8 @@ static_assert(NSIGBUS == 5);
static_assert(NSIGTRAP == 6);
static_assert(NSIGCHLD == 6);
static_assert(NSIGSYS == 2);
+static_assert(sizeof(siginfo_t) == 128);
+static_assert(__alignof__(siginfo_t) == 8);
static_assert(offsetof(siginfo_t, si_signo) == 0x00);
static_assert(offsetof(siginfo_t, si_errno) == 0x04);
static_assert(offsetof(siginfo_t, si_code) == 0x08);
diff --git a/arch/x86/kernel/signal_compat.c b/arch/x86/kernel/signal_compat.c
index 0e5d0a7e203b..e735bc129331 100644
--- a/arch/x86/kernel/signal_compat.c
+++ b/arch/x86/kernel/signal_compat.c
@@ -34,7 +34,13 @@ static inline void signal_compat_build_tests(void)
BUILD_BUG_ON(NSIGSYS != 2);
/* This is part of the ABI and can never change in size: */
+ BUILD_BUG_ON(sizeof(siginfo_t) != 128);
BUILD_BUG_ON(sizeof(compat_siginfo_t) != 128);
+
+ /* This is a part of the ABI and can never change in alignment */
+ BUILD_BUG_ON(__alignof__(siginfo_t) != 8);
+ BUILD_BUG_ON(__alignof__(compat_siginfo_t) != 4);
+
/*
* The offsets of all the (unioned) si_fields are fixed
* in the ABI, of course. Make sure none of them ever
diff --git a/include/uapi/asm-generic/siginfo.h b/include/uapi/asm-generic/siginfo.h
index 03d6f6d2c1fe..91c80d0c10c5 100644
--- a/include/uapi/asm-generic/siginfo.h
+++ b/include/uapi/asm-generic/siginfo.h
@@ -29,6 +29,11 @@ typedef union sigval {
#define __ARCH_SI_ATTRIBUTES
#endif
+/*
+ * Be careful when extending this union. On 32bit siginfo_t is 32bit
+ * aligned. Which means that a 64bit field or any other field that
+ * would increase the alignment of siginfo_t will break the ABI.
+ */
union __sifields {
/* kill() */
struct {
--
2.30.1
On Wed, 5 May 2021 at 16:11, Eric W. Beiderman <[email protected]> wrote:
> From: "Eric W. Biederman" <[email protected]>
>
> Update the static assertions about siginfo_t to also describe
> it's alignment and size.
>
> While investigating if it was possible to add a 64bit field into
> siginfo_t[1] it became apparent that the alignment of siginfo_t
> is as much a part of the ABI as the size of the structure.
>
> If the alignment changes siginfo_t when embedded in another structure
> can move to a different offset. Which is not acceptable from an ABI
> structure.
>
> So document that fact and add static assertions to notify developers
> if they change change the alignment by accident.
>
> [1] https://lkml.kernel.org/r/[email protected]
> Signed-off-by: "Eric W. Biederman" <[email protected]>
Acked-by: Marco Elver <[email protected]>
> ---
> arch/arm/kernel/signal.c | 2 ++
> arch/arm64/kernel/signal.c | 2 ++
> arch/arm64/kernel/signal32.c | 2 ++
> arch/sparc/kernel/signal32.c | 2 ++
> arch/sparc/kernel/signal_64.c | 2 ++
> arch/x86/kernel/signal_compat.c | 6 ++++++
> include/uapi/asm-generic/siginfo.h | 5 +++++
> 7 files changed, 21 insertions(+)
>
> diff --git a/arch/arm/kernel/signal.c b/arch/arm/kernel/signal.c
> index 2dac5d2c5cf6..643bcb0f091b 100644
> --- a/arch/arm/kernel/signal.c
> +++ b/arch/arm/kernel/signal.c
> @@ -737,6 +737,8 @@ static_assert(NSIGBUS == 5);
> static_assert(NSIGTRAP == 6);
> static_assert(NSIGCHLD == 6);
> static_assert(NSIGSYS == 2);
> +static_assert(sizeof(siginfo_t) == 128);
> +static_assert(__alignof__(siginfo_t) == 4);
> static_assert(offsetof(siginfo_t, si_signo) == 0x00);
> static_assert(offsetof(siginfo_t, si_errno) == 0x04);
> static_assert(offsetof(siginfo_t, si_code) == 0x08);
> diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
> index af8bd2af1298..ad4bd27fc044 100644
> --- a/arch/arm64/kernel/signal.c
> +++ b/arch/arm64/kernel/signal.c
> @@ -985,6 +985,8 @@ static_assert(NSIGBUS == 5);
> static_assert(NSIGTRAP == 6);
> static_assert(NSIGCHLD == 6);
> static_assert(NSIGSYS == 2);
> +static_assert(sizeof(siginfo_t) == 128);
Would using SI_MAX_SIZE be appropriate? Perhaps not.. in case somebody
changes it, given these static asserts are meant to double-check.
I leave it to you to decide what makes more sense.
> +static_assert(__alignof__(siginfo_t) == 8);
> static_assert(offsetof(siginfo_t, si_signo) == 0x00);
> static_assert(offsetof(siginfo_t, si_errno) == 0x04);
> static_assert(offsetof(siginfo_t, si_code) == 0x08);
> diff --git a/arch/arm64/kernel/signal32.c b/arch/arm64/kernel/signal32.c
> index b6afb646515f..ee6c7484e130 100644
> --- a/arch/arm64/kernel/signal32.c
> +++ b/arch/arm64/kernel/signal32.c
> @@ -469,6 +469,8 @@ static_assert(NSIGBUS == 5);
> static_assert(NSIGTRAP == 6);
> static_assert(NSIGCHLD == 6);
> static_assert(NSIGSYS == 2);
> +static_assert(sizeof(compat_siginfo_t) == 128);
> +static_assert(__alignof__(compat_siginfo_t) == 4);
> static_assert(offsetof(compat_siginfo_t, si_signo) == 0x00);
> static_assert(offsetof(compat_siginfo_t, si_errno) == 0x04);
> static_assert(offsetof(compat_siginfo_t, si_code) == 0x08);
> diff --git a/arch/sparc/kernel/signal32.c b/arch/sparc/kernel/signal32.c
> index 778ed5c26d4a..32b977f253e3 100644
> --- a/arch/sparc/kernel/signal32.c
> +++ b/arch/sparc/kernel/signal32.c
> @@ -757,6 +757,8 @@ static_assert(NSIGBUS == 5);
> static_assert(NSIGTRAP == 6);
> static_assert(NSIGCHLD == 6);
> static_assert(NSIGSYS == 2);
> +static_assert(sizeof(compat_siginfo_t) == 128);
> +static_assert(__alignof__(compat_siginfo_t) == 4);
> static_assert(offsetof(compat_siginfo_t, si_signo) == 0x00);
> static_assert(offsetof(compat_siginfo_t, si_errno) == 0x04);
> static_assert(offsetof(compat_siginfo_t, si_code) == 0x08);
> diff --git a/arch/sparc/kernel/signal_64.c b/arch/sparc/kernel/signal_64.c
> index c9bbf5f29078..e9dda9db156c 100644
> --- a/arch/sparc/kernel/signal_64.c
> +++ b/arch/sparc/kernel/signal_64.c
> @@ -567,6 +567,8 @@ static_assert(NSIGBUS == 5);
> static_assert(NSIGTRAP == 6);
> static_assert(NSIGCHLD == 6);
> static_assert(NSIGSYS == 2);
> +static_assert(sizeof(siginfo_t) == 128);
> +static_assert(__alignof__(siginfo_t) == 8);
> static_assert(offsetof(siginfo_t, si_signo) == 0x00);
> static_assert(offsetof(siginfo_t, si_errno) == 0x04);
> static_assert(offsetof(siginfo_t, si_code) == 0x08);
> diff --git a/arch/x86/kernel/signal_compat.c b/arch/x86/kernel/signal_compat.c
> index 0e5d0a7e203b..e735bc129331 100644
> --- a/arch/x86/kernel/signal_compat.c
> +++ b/arch/x86/kernel/signal_compat.c
> @@ -34,7 +34,13 @@ static inline void signal_compat_build_tests(void)
> BUILD_BUG_ON(NSIGSYS != 2);
>
> /* This is part of the ABI and can never change in size: */
> + BUILD_BUG_ON(sizeof(siginfo_t) != 128);
> BUILD_BUG_ON(sizeof(compat_siginfo_t) != 128);
> +
> + /* This is a part of the ABI and can never change in alignment */
> + BUILD_BUG_ON(__alignof__(siginfo_t) != 8);
> + BUILD_BUG_ON(__alignof__(compat_siginfo_t) != 4);
> +
> /*
> * The offsets of all the (unioned) si_fields are fixed
> * in the ABI, of course. Make sure none of them ever
> diff --git a/include/uapi/asm-generic/siginfo.h b/include/uapi/asm-generic/siginfo.h
> index 03d6f6d2c1fe..91c80d0c10c5 100644
> --- a/include/uapi/asm-generic/siginfo.h
> +++ b/include/uapi/asm-generic/siginfo.h
> @@ -29,6 +29,11 @@ typedef union sigval {
> #define __ARCH_SI_ATTRIBUTES
> #endif
>
> +/*
> + * Be careful when extending this union. On 32bit siginfo_t is 32bit
> + * aligned. Which means that a 64bit field or any other field that
> + * would increase the alignment of siginfo_t will break the ABI.
> + */
> union __sifields {
> /* kill() */
> struct {
> --
> 2.30.1
>