On 2025-08-18 12:16, Dmytro Prokopchuk1 wrote:
On 8/14/25 23:43, Nicola Vetrini wrote:
On 2025-08-14 10:36, Jan Beulich wrote:
On 13.08.2025 20:27, Dmytro Prokopchuk1 wrote:
...
from `vaddr_t' (that is `unsigned long') to `switch_ttbr_fn*' (that
is `void(*)(unsigned long)')
Signed-off-by: Dmytro Prokopchuk <dmytro_prokopch...@epam.com>
---
This is just a RFC patch.
The commit message is not important at this stage.
I am seeking comments regarding this case.
Thanks.
---
automation/eclair_analysis/ECLAIR/deviations.ecl | 8 ++++++++
docs/misra/deviations.rst | 10 ++++++++++
docs/misra/rules.rst | 8 +++++++-
xen/arch/arm/arm64/mmu/mm.c | 2 ++
4 files changed, 27 insertions(+), 1 deletion(-)
diff --git a/automation/eclair_analysis/ECLAIR/deviations.ecl b/
automation/eclair_analysis/ECLAIR/deviations.ecl
index ebce1ceab9..f9fd6076b7 100644
--- a/automation/eclair_analysis/ECLAIR/deviations.ecl
+++ b/automation/eclair_analysis/ECLAIR/deviations.ecl
@@ -365,6 +365,14 @@ constant expressions are required.\""
}
-doc_end
+-doc_begin="The conversion from unsigned long to a function pointer
does not lose any information, provided that the source type has
enough bits to restore it."
+-config=MC3A2.R11.1,casts+={safe,
+ "from(type(canonical(builtin(unsigned long))))
+ &&to(type(canonical(__function_pointer_types)))
+ &&relation(definitely_preserves_value)"
+}
+-doc_end
+
This check is not quite targeted at this situation, as the behaviour
of
different compilers is a bit of a grey area (even GCC, though that
works
in practice). The relation is mostly aimed at testing whether the
pointer are represented using the same number of bits as unsigned long
(which happens to be the case fortunately).
Hi Nicola.
Well, we're telling Eclair the conversion types from() and to(), but
can
Eclair determine their sizes (in bits) for particular architecture?
I mean, is it possible to avoid this "sizeof(unsigned long) ==
sizeof(void (*)())" in source code using only Eclair configs?
Dmytro.
Unfortunately no. ECLAIR knowns the number of bytes used to represent
pointer and unsigned long, but what it cannot tell is whether the bits
are preserved after being converted. What we can do, as it was done
here, is provide a written justification that this is indeed the case
for the toolchain we care about (GCC in the specific case). I suggest
having both the config and the assertion to be extra sure that the
assumption is never broken (despite being very unlikely).
-doc_begin="The conversion from a function pointer to a boolean has
a well-known semantics that do not lead to unexpected behaviour."
-config=MC3A2.R11.1,casts+={safe,
"from(type(canonical(__function_pointer_types)))
diff --git a/docs/misra/deviations.rst b/docs/misra/deviations.rst
index 3c46a1e47a..27848602f6 100644
--- a/docs/misra/deviations.rst
+++ b/docs/misra/deviations.rst
@@ -348,6 +348,16 @@ Deviations related to MISRA C:2012 Rules:
to store it.
- Tagged as `safe` for ECLAIR.
+ * - R11.1
+ - The conversion from unsigned long to a function pointer does
not lose any
+ information or violate type safety assumptions if the
unsigned long type
+ is guaranteed to be at least as large as a function pointer.
This ensures
+ that the function pointer address can be fully represented
without
+ truncation or corruption. Macro BUILD_BUG_ON can be
integrated into the
+ build system to confirm that 'sizeof(unsigned long) >=
sizeof(void (*)())'
+ on all target platforms.
If sizeof(unsigned long) > sizeof(void (*)()), there is loss of
information.
Unless (not said here) the unsigned long value itself is the result
of
converting a function pointer to unsigned long. Whether all of that
together
can be properly expressed to Eclair I don't know. Hence, as Teddy
already
suggested, == may want specifying instead.
+1; it might be worth to add both the eclair config and the
BUILD_BUG_ON, noting that neither is sufficient on its own: unless the
compiler guarantees not to fiddle with the value is unaltered when
cast
back and forth all checks on the number of bits are moot.
--- a/xen/arch/arm/arm64/mmu/mm.c
+++ b/xen/arch/arm/arm64/mmu/mm.c
@@ -150,6 +150,7 @@ void __init relocate_and_switch_ttbr(uint64_t
ttbr)
vaddr_t id_addr = virt_to_maddr(relocate_xen);
relocate_xen_fn *fn = (relocate_xen_fn *)id_addr;
lpae_t pte;
+ BUILD_BUG_ON(sizeof(unsigned long) < sizeof(fn));
/* Enable the identity mapping in the boot page tables */
update_identity_mapping(true);
@@ -178,6 +179,7 @@ void __init switch_ttbr(uint64_t ttbr)
vaddr_t id_addr = virt_to_maddr(switch_ttbr_id);
switch_ttbr_fn *fn = (switch_ttbr_fn *)id_addr;
lpae_t pte;
+ BUILD_BUG_ON(sizeof(unsigned long) < sizeof(fn));
/* Enable the identity mapping in the boot page tables */
update_identity_mapping(true);
BUILD_BUG_ON() is a statement, not a declaration, and hence wants
grouping
as such. Question is whether we indeed want to sprinkle such checks
all
over the code base. (I expect the two cases here aren't all we have.)
+1 as well. I would expect such check to live e.g. in compiler.h or
any
similarly general header, since this is a widespread and largely arch-
neutral property that Xen wants to be always true I believe.
--
Nicola Vetrini, B.Sc.
Software Engineer
BUGSENG (https://bugseng.com)
LinkedIn: https://www.linkedin.com/in/nicola-vetrini-a42471253
