Discussion:
[PATCH] bfd: xtensa: ignore overflow in hight part of const16 relocation
(too old to reply)
Max Filippov
2018-12-07 21:17:29 UTC
Permalink
32-bit constants loaded by two const16 opcodes that involve relocation
(e.g. calculated as a sum of a symbol and a constant) may overflow,
resulting in linking error with the following message:

dangerous relocation: const16: cannot encode: (_start+0x70000000)

They whould wrap around instead.

bfd/
2018-12-07 Max Filippov <***@gmail.com>

* elf32-xtensa.c (elf_xtensa_do_reloc): Only use bits 16..31 of
the relocated value for the high part const16 immediate.
---
bfd/elf32-xtensa.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/bfd/elf32-xtensa.c b/bfd/elf32-xtensa.c
index cf085b7b0751..de960cd3b8fc 100644
--- a/bfd/elf32-xtensa.c
+++ b/bfd/elf32-xtensa.c
@@ -1957,8 +1957,9 @@ elf_xtensa_do_reloc (reloc_howto_type *howto,
}
else if (opcode == get_const16_opcode ())
{
- /* ALT used for high 16 bits. */
- newval = relocation >> 16;
+ /* ALT used for high 16 bits.
+ Ignore 32-bit overflow. */
+ newval = (relocation >> 16) & 0xffff;
opnd = 1;
}
else
--
2.11.0
a***@gmail.com
2018-12-07 21:31:00 UTC
Permalink
Post by Max Filippov
32-bit constants loaded by two const16 opcodes that involve relocation
(e.g. calculated as a sum of a symbol and a constant) may overflow,
dangerous relocation: const16: cannot encode: (_start+0x70000000)
They whould wrap around instead.
bfd/
* elf32-xtensa.c (elf_xtensa_do_reloc): Only use bits 16..31 of
the relocated value for the high part const16 immediate.
This is generally OK. I would probably change the comment to, "Ignore
high sixteen bits". The way it is worded now it sounds like a 32-bit
overflow, so more than ~4GB.
Post by Max Filippov
---
bfd/elf32-xtensa.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)
diff --git a/bfd/elf32-xtensa.c b/bfd/elf32-xtensa.c
index cf085b7b0751..de960cd3b8fc 100644
--- a/bfd/elf32-xtensa.c
+++ b/bfd/elf32-xtensa.c
@@ -1957,8 +1957,9 @@ elf_xtensa_do_reloc (reloc_howto_type *howto,
}
else if (opcode == get_const16_opcode ())
{
- /* ALT used for high 16 bits. */
- newval = relocation >> 16;
+ /* ALT used for high 16 bits.
+ Ignore 32-bit overflow. */
+ newval = (relocation >> 16) & 0xffff;
opnd = 1;
}
else
--
2.11.0
Max Filippov
2018-12-07 21:38:17 UTC
Permalink
Post by a***@gmail.com
Post by Max Filippov
32-bit constants loaded by two const16 opcodes that involve relocation
(e.g. calculated as a sum of a symbol and a constant) may overflow,
dangerous relocation: const16: cannot encode: (_start+0x70000000)
They whould wrap around instead.
bfd/
* elf32-xtensa.c (elf_xtensa_do_reloc): Only use bits 16..31 of
the relocated value for the high part const16 immediate.
This is generally OK. I would probably change the comment to, "Ignore
high sixteen bits". The way it is worded now it sounds like a 32-bit
overflow, so more than ~4GB.
It is a 32-bit overflow of the relocated value, and when the value is
shifted right by 16 to extract immediate field for the const16 it doesn't
fit into that 16-bit field.
So the fix is to ignore all bits above the bit 31 of the relocated value,
i.e. to only use bits 16..31 for the const16 immediate.
--
Thanks.
-- Max
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