=== Motivation ===
Similar to reading from CO-RE bitfields, we need a CO-RE aware bitfield
writing wrapper to make the verifier happy.
Two alternatives to this approach are:
1. Use the upcoming `preserve_static_offset` [0] attribute to disable
CO-RE on specific structs.
2. Use broader byte-sized writes to write to bitfields.
(1) is a bit hard to use. It requires specific and not-very-obvious
annotations to bpftool generated vmlinux.h. It's also not generally
available in released LLVM versions yet.
(2) makes the code quite hard to read and write. And especially if
BPF_CORE_READ_BITFIELD() is already being used, it makes more sense to
to have an inverse helper for writing.
=== Implementation details ===
Since the logic is a bit non-obvious, I thought it would be helpful
to explain exactly what's going on.
To start, it helps by explaining what LSHIFT_U64 (lshift) and RSHIFT_U64
(rshift) is designed to mean. Consider the core of the
BPF_CORE_READ_BITFIELD() algorithm:
val <<= __CORE_RELO(s, field, LSHIFT_U64);
val = val >> __CORE_RELO(s, field, RSHIFT_U64);
Basically what happens is we lshift to clear the non-relevant (blank)
higher order bits. Then we rshift to bring the relevant bits (bitfield)
down to LSB position (while also clearing blank lower order bits). To
illustrate:
Start: ........XXX......
Lshift: XXX......00000000
Rshift: 00000000000000XXX
where `.` means blank bit, `0` means 0 bit, and `X` means bitfield bit.
After the two operations, the bitfield is ready to be interpreted as a
regular integer.
Next, we want to build an alternative (but more helpful) mental model
on lshift and rshift. That is, to consider:
* rshift as the total number of blank bits in the u64
* lshift as number of blank bits left of the bitfield in the u64
Take a moment to consider why that is true by consulting the above
diagram.
With this insight, we can how define the following relationship:
bitfield
_
| |
0.....00XXX0...00
| | | |
|______| | |
lshift | |
|____|
(rshift - lshift)
That is, we know the number of higher order blank bits is just lshift.
And the number of lower order blank bits is (rshift - lshift).
Finally, we can examine the core of the write side algorithm:
mask = (~0ULL << rshift) >> lshift; // 1
nval = new_val; // 2
nval = (nval << rpad) & mask; // 3
val = (val & ~mask) | nval; // 4
(1): Compute a mask where the set bits are the bitfield bits. The first
left shift zeros out exactly the number of blank bits, leaving a
bitfield sized set of 1s. The subsequent right shift inserts the
correct amount of higher order blank bits.
(2): Place the new value into a word sized container, nval.
(3): Place nval at the correct bit position and mask out blank bits.
(4): Mix the bitfield in with original surrounding blank bits.
[0]: https://reviews.llvm.org/D133361
Co-authored-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Co-authored-by: Jonathan Lemon <jlemon@aviatrix.com>
Signed-off-by: Jonathan Lemon <jlemon@aviatrix.com>
Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
---
tools/lib/bpf/bpf_core_read.h | 34 ++++++++++++++++++++++++++++++++++
1 file changed, 34 insertions(+)
diff --git a/tools/lib/bpf/bpf_core_read.h b/tools/lib/bpf/bpf_core_read.h
index 1ac57bb7ac55..a7ffb80e3539 100644
--- a/tools/lib/bpf/bpf_core_read.h
+++ b/tools/lib/bpf/bpf_core_read.h
@@ -111,6 +111,40 @@ enum bpf_enum_value_kind {
val; \
})
+/*
+ * Write to a bitfield, identified by s->field.
+ * This is the inverse of BPF_CORE_WRITE_BITFIELD().
+ */
+#define BPF_CORE_WRITE_BITFIELD(s, field, new_val) ({ \
+ void *p = (void *)s + __CORE_RELO(s, field, BYTE_OFFSET); \
+ unsigned int byte_size = __CORE_RELO(s, field, BYTE_SIZE); \
+ unsigned int lshift = __CORE_RELO(s, field, LSHIFT_U64); \
+ unsigned int rshift = __CORE_RELO(s, field, RSHIFT_U64); \
+ unsigned int rpad = rshift - lshift; \
+ unsigned long long nval, mask, val; \
+ \
+ asm volatile("" : "+r"(p)); \
+ \
+ switch (byte_size) { \
+ case 1: val = *(unsigned char *)p; break; \
+ case 2: val = *(unsigned short *)p; break; \
+ case 4: val = *(unsigned int *)p; break; \
+ case 8: val = *(unsigned long long *)p; break; \
+ } \
+ \
+ mask = (~0ULL << rshift) >> lshift; \
+ nval = new_val; \
+ nval = (nval << rpad) & mask; \
+ val = (val & ~mask) | nval; \
+ \
+ switch (byte_size) { \
+ case 1: *(unsigned char *)p = val; break; \
+ case 2: *(unsigned short *)p = val; break; \
+ case 4: *(unsigned int *)p = val; break; \
+ case 8: *(unsigned long long *)p = val; break; \
+ } \
+})
+
#define ___bpf_field_ref1(field) (field)
#define ___bpf_field_ref2(type, field) (((typeof(type) *)0)->field)
#define ___bpf_field_ref(args...) \
--
2.42.1On Fri, Dec 1, 2023 at 12:24 PM Daniel Xu <dxu@dxuuu.xyz> wrote:
>
> === Motivation ===
>
> Similar to reading from CO-RE bitfields, we need a CO-RE aware bitfield
> writing wrapper to make the verifier happy.
>
> Two alternatives to this approach are:
>
> 1. Use the upcoming `preserve_static_offset` [0] attribute to disable
> CO-RE on specific structs.
> 2. Use broader byte-sized writes to write to bitfields.
>
> (1) is a bit hard to use. It requires specific and not-very-obvious
> annotations to bpftool generated vmlinux.h. It's also not generally
> available in released LLVM versions yet.
>
> (2) makes the code quite hard to read and write. And especially if
> BPF_CORE_READ_BITFIELD() is already being used, it makes more sense to
> to have an inverse helper for writing.
>
> === Implementation details ===
>
> Since the logic is a bit non-obvious, I thought it would be helpful
> to explain exactly what's going on.
>
> To start, it helps by explaining what LSHIFT_U64 (lshift) and RSHIFT_U64
> (rshift) is designed to mean. Consider the core of the
> BPF_CORE_READ_BITFIELD() algorithm:
>
> val <<= __CORE_RELO(s, field, LSHIFT_U64);
> val = val >> __CORE_RELO(s, field, RSHIFT_U64);
nit: indentation is off?
>
> Basically what happens is we lshift to clear the non-relevant (blank)
> higher order bits. Then we rshift to bring the relevant bits (bitfield)
> down to LSB position (while also clearing blank lower order bits). To
> illustrate:
>
> Start: ........XXX......
> Lshift: XXX......00000000
> Rshift: 00000000000000XXX
>
> where `.` means blank bit, `0` means 0 bit, and `X` means bitfield bit.
>
> After the two operations, the bitfield is ready to be interpreted as a
> regular integer.
>
> Next, we want to build an alternative (but more helpful) mental model
> on lshift and rshift. That is, to consider:
>
> * rshift as the total number of blank bits in the u64
> * lshift as number of blank bits left of the bitfield in the u64
>
> Take a moment to consider why that is true by consulting the above
> diagram.
>
> With this insight, we can how define the following relationship:
>
> bitfield
> _
> | |
> 0.....00XXX0...00
> | | | |
> |______| | |
> lshift | |
> |____|
> (rshift - lshift)
>
> That is, we know the number of higher order blank bits is just lshift.
> And the number of lower order blank bits is (rshift - lshift).
>
Nice diagrams and description, thanks!
> Finally, we can examine the core of the write side algorithm:
>
> mask = (~0ULL << rshift) >> lshift; // 1
> nval = new_val; // 2
> nval = (nval << rpad) & mask; // 3
> val = (val & ~mask) | nval; // 4
>
> (1): Compute a mask where the set bits are the bitfield bits. The first
> left shift zeros out exactly the number of blank bits, leaving a
> bitfield sized set of 1s. The subsequent right shift inserts the
> correct amount of higher order blank bits.
> (2): Place the new value into a word sized container, nval.
> (3): Place nval at the correct bit position and mask out blank bits.
> (4): Mix the bitfield in with original surrounding blank bits.
>
> [0]: https://reviews.llvm.org/D133361
> Co-authored-by: Eduard Zingerman <eddyz87@gmail.com>
> Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
> Co-authored-by: Jonathan Lemon <jlemon@aviatrix.com>
> Signed-off-by: Jonathan Lemon <jlemon@aviatrix.com>
> Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
> ---
> tools/lib/bpf/bpf_core_read.h | 34 ++++++++++++++++++++++++++++++++++
> 1 file changed, 34 insertions(+)
>
> diff --git a/tools/lib/bpf/bpf_core_read.h b/tools/lib/bpf/bpf_core_read.h
> index 1ac57bb7ac55..a7ffb80e3539 100644
> --- a/tools/lib/bpf/bpf_core_read.h
> +++ b/tools/lib/bpf/bpf_core_read.h
> @@ -111,6 +111,40 @@ enum bpf_enum_value_kind {
> val; \
> })
>
> +/*
> + * Write to a bitfield, identified by s->field.
> + * This is the inverse of BPF_CORE_WRITE_BITFIELD().
> + */
> +#define BPF_CORE_WRITE_BITFIELD(s, field, new_val) ({ \
> + void *p = (void *)s + __CORE_RELO(s, field, BYTE_OFFSET); \
> + unsigned int byte_size = __CORE_RELO(s, field, BYTE_SIZE); \
> + unsigned int lshift = __CORE_RELO(s, field, LSHIFT_U64); \
> + unsigned int rshift = __CORE_RELO(s, field, RSHIFT_U64); \
> + unsigned int rpad = rshift - lshift; \
> + unsigned long long nval, mask, val; \
> + \
> + asm volatile("" : "+r"(p)); \
> + \
> + switch (byte_size) { \
> + case 1: val = *(unsigned char *)p; break; \
> + case 2: val = *(unsigned short *)p; break; \
> + case 4: val = *(unsigned int *)p; break; \
> + case 8: val = *(unsigned long long *)p; break; \
> + } \
> + \
> + mask = (~0ULL << rshift) >> lshift; \
> + nval = new_val; \
> + nval = (nval << rpad) & mask; \
> + val = (val & ~mask) | nval; \
I'd simplify it to not need nval at all
val = (val & ~mask) | ((new_val << rpad) & mask);
I actually find it easier to follow and make sure we are not doing
anything unexpected. First part before |, we take old value and clear
bits we are about to set, second part after |, we take bitfield value,
shift it in position, and just in case mask it out if it's too big to
fit. Combine, done.
Other than that, it looks good.
> + \
> + switch (byte_size) { \
> + case 1: *(unsigned char *)p = val; break; \
> + case 2: *(unsigned short *)p = val; break; \
> + case 4: *(unsigned int *)p = val; break; \
> + case 8: *(unsigned long long *)p = val; break; \
> + } \
> +})
> +
> #define ___bpf_field_ref1(field) (field)
> #define ___bpf_field_ref2(type, field) (((typeof(type) *)0)->field)
> #define ___bpf_field_ref(args...) \
> --
> 2.42.1
>
On Fri, Dec 01, 2023 at 03:49:30PM -0800, Andrii Nakryiko wrote:
> On Fri, Dec 1, 2023 at 12:24 PM Daniel Xu <dxu@dxuuu.xyz> wrote:
> >
> > === Motivation ===
> >
> > Similar to reading from CO-RE bitfields, we need a CO-RE aware bitfield
> > writing wrapper to make the verifier happy.
> >
> > Two alternatives to this approach are:
> >
> > 1. Use the upcoming `preserve_static_offset` [0] attribute to disable
> > CO-RE on specific structs.
> > 2. Use broader byte-sized writes to write to bitfields.
> >
> > (1) is a bit hard to use. It requires specific and not-very-obvious
> > annotations to bpftool generated vmlinux.h. It's also not generally
> > available in released LLVM versions yet.
> >
> > (2) makes the code quite hard to read and write. And especially if
> > BPF_CORE_READ_BITFIELD() is already being used, it makes more sense to
> > to have an inverse helper for writing.
> >
> > === Implementation details ===
> >
> > Since the logic is a bit non-obvious, I thought it would be helpful
> > to explain exactly what's going on.
> >
> > To start, it helps by explaining what LSHIFT_U64 (lshift) and RSHIFT_U64
> > (rshift) is designed to mean. Consider the core of the
> > BPF_CORE_READ_BITFIELD() algorithm:
> >
> > val <<= __CORE_RELO(s, field, LSHIFT_U64);
> > val = val >> __CORE_RELO(s, field, RSHIFT_U64);
>
> nit: indentation is off?
Oops, it's cuz I only deleted the SIGNED check. Will fix.
>
> >
> > Basically what happens is we lshift to clear the non-relevant (blank)
> > higher order bits. Then we rshift to bring the relevant bits (bitfield)
> > down to LSB position (while also clearing blank lower order bits). To
> > illustrate:
> >
> > Start: ........XXX......
> > Lshift: XXX......00000000
> > Rshift: 00000000000000XXX
> >
> > where `.` means blank bit, `0` means 0 bit, and `X` means bitfield bit.
> >
> > After the two operations, the bitfield is ready to be interpreted as a
> > regular integer.
> >
> > Next, we want to build an alternative (but more helpful) mental model
> > on lshift and rshift. That is, to consider:
> >
> > * rshift as the total number of blank bits in the u64
> > * lshift as number of blank bits left of the bitfield in the u64
> >
> > Take a moment to consider why that is true by consulting the above
> > diagram.
> >
> > With this insight, we can how define the following relationship:
> >
> > bitfield
> > _
> > | |
> > 0.....00XXX0...00
> > | | | |
> > |______| | |
> > lshift | |
> > |____|
> > (rshift - lshift)
> >
> > That is, we know the number of higher order blank bits is just lshift.
> > And the number of lower order blank bits is (rshift - lshift).
> >
>
> Nice diagrams and description, thanks!
Thanks!
>
> > Finally, we can examine the core of the write side algorithm:
> >
> > mask = (~0ULL << rshift) >> lshift; // 1
> > nval = new_val; // 2
> > nval = (nval << rpad) & mask; // 3
> > val = (val & ~mask) | nval; // 4
> >
> > (1): Compute a mask where the set bits are the bitfield bits. The first
> > left shift zeros out exactly the number of blank bits, leaving a
> > bitfield sized set of 1s. The subsequent right shift inserts the
> > correct amount of higher order blank bits.
> > (2): Place the new value into a word sized container, nval.
> > (3): Place nval at the correct bit position and mask out blank bits.
> > (4): Mix the bitfield in with original surrounding blank bits.
> >
> > [0]: https://reviews.llvm.org/D133361
> > Co-authored-by: Eduard Zingerman <eddyz87@gmail.com>
> > Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
> > Co-authored-by: Jonathan Lemon <jlemon@aviatrix.com>
> > Signed-off-by: Jonathan Lemon <jlemon@aviatrix.com>
> > Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
> > ---
> > tools/lib/bpf/bpf_core_read.h | 34 ++++++++++++++++++++++++++++++++++
> > 1 file changed, 34 insertions(+)
> >
> > diff --git a/tools/lib/bpf/bpf_core_read.h b/tools/lib/bpf/bpf_core_read.h
> > index 1ac57bb7ac55..a7ffb80e3539 100644
> > --- a/tools/lib/bpf/bpf_core_read.h
> > +++ b/tools/lib/bpf/bpf_core_read.h
> > @@ -111,6 +111,40 @@ enum bpf_enum_value_kind {
> > val; \
> > })
> >
> > +/*
> > + * Write to a bitfield, identified by s->field.
> > + * This is the inverse of BPF_CORE_WRITE_BITFIELD().
> > + */
> > +#define BPF_CORE_WRITE_BITFIELD(s, field, new_val) ({ \
> > + void *p = (void *)s + __CORE_RELO(s, field, BYTE_OFFSET); \
> > + unsigned int byte_size = __CORE_RELO(s, field, BYTE_SIZE); \
> > + unsigned int lshift = __CORE_RELO(s, field, LSHIFT_U64); \
> > + unsigned int rshift = __CORE_RELO(s, field, RSHIFT_U64); \
> > + unsigned int rpad = rshift - lshift; \
> > + unsigned long long nval, mask, val; \
> > + \
> > + asm volatile("" : "+r"(p)); \
> > + \
> > + switch (byte_size) { \
> > + case 1: val = *(unsigned char *)p; break; \
> > + case 2: val = *(unsigned short *)p; break; \
> > + case 4: val = *(unsigned int *)p; break; \
> > + case 8: val = *(unsigned long long *)p; break; \
> > + } \
> > + \
> > + mask = (~0ULL << rshift) >> lshift; \
> > + nval = new_val; \
> > + nval = (nval << rpad) & mask; \
> > + val = (val & ~mask) | nval; \
>
> I'd simplify it to not need nval at all
>
> val = (val & ~mask) | ((new_val << rpad) & mask);
>
> I actually find it easier to follow and make sure we are not doing
> anything unexpected. First part before |, we take old value and clear
> bits we are about to set, second part after |, we take bitfield value,
> shift it in position, and just in case mask it out if it's too big to
> fit. Combine, done.
>
> Other than that, it looks good.
I mostly left it there for the cast. Cuz injecting the `unsigned long
long` cast made the line really long. How about this instead?
diff --git a/tools/lib/bpf/bpf_core_read.h b/tools/lib/bpf/bpf_core_read.h
index a7ffb80e3539..7325a12692a3 100644
--- a/tools/lib/bpf/bpf_core_read.h
+++ b/tools/lib/bpf/bpf_core_read.h
@@ -120,8 +120,8 @@ enum bpf_enum_value_kind {
unsigned int byte_size = __CORE_RELO(s, field, BYTE_SIZE); \
unsigned int lshift = __CORE_RELO(s, field, LSHIFT_U64); \
unsigned int rshift = __CORE_RELO(s, field, RSHIFT_U64); \
+ unsigned long long mask, val, nval = new_val; \
unsigned int rpad = rshift - lshift; \
- unsigned long long nval, mask, val; \
\
asm volatile("" : "+r"(p)); \
\
@@ -133,9 +133,7 @@ enum bpf_enum_value_kind {
} \
\
mask = (~0ULL << rshift) >> lshift; \
- nval = new_val; \
- nval = (nval << rpad) & mask; \
- val = (val & ~mask) | nval; \
+ val = (val & ~mask) | ((nval << rpad) & mask); \
\
switch (byte_size) { \
case 1: *(unsigned char *)p = val; break; \
Thanks,
Daniel
On Fri, Dec 1, 2023 at 4:13 PM Daniel Xu <dxu@dxuuu.xyz> wrote:
>
> On Fri, Dec 01, 2023 at 03:49:30PM -0800, Andrii Nakryiko wrote:
> > On Fri, Dec 1, 2023 at 12:24 PM Daniel Xu <dxu@dxuuu.xyz> wrote:
> > >
> > > === Motivation ===
> > >
> > > Similar to reading from CO-RE bitfields, we need a CO-RE aware bitfield
> > > writing wrapper to make the verifier happy.
> > >
> > > Two alternatives to this approach are:
> > >
> > > 1. Use the upcoming `preserve_static_offset` [0] attribute to disable
> > > CO-RE on specific structs.
> > > 2. Use broader byte-sized writes to write to bitfields.
> > >
> > > (1) is a bit hard to use. It requires specific and not-very-obvious
> > > annotations to bpftool generated vmlinux.h. It's also not generally
> > > available in released LLVM versions yet.
> > >
> > > (2) makes the code quite hard to read and write. And especially if
> > > BPF_CORE_READ_BITFIELD() is already being used, it makes more sense to
> > > to have an inverse helper for writing.
> > >
> > > === Implementation details ===
> > >
> > > Since the logic is a bit non-obvious, I thought it would be helpful
> > > to explain exactly what's going on.
> > >
> > > To start, it helps by explaining what LSHIFT_U64 (lshift) and RSHIFT_U64
> > > (rshift) is designed to mean. Consider the core of the
> > > BPF_CORE_READ_BITFIELD() algorithm:
> > >
> > > val <<= __CORE_RELO(s, field, LSHIFT_U64);
> > > val = val >> __CORE_RELO(s, field, RSHIFT_U64);
> >
> > nit: indentation is off?
>
> Oops, it's cuz I only deleted the SIGNED check. Will fix.
> >
> > >
> > > Basically what happens is we lshift to clear the non-relevant (blank)
> > > higher order bits. Then we rshift to bring the relevant bits (bitfield)
> > > down to LSB position (while also clearing blank lower order bits). To
> > > illustrate:
> > >
> > > Start: ........XXX......
> > > Lshift: XXX......00000000
> > > Rshift: 00000000000000XXX
> > >
> > > where `.` means blank bit, `0` means 0 bit, and `X` means bitfield bit.
> > >
> > > After the two operations, the bitfield is ready to be interpreted as a
> > > regular integer.
> > >
> > > Next, we want to build an alternative (but more helpful) mental model
> > > on lshift and rshift. That is, to consider:
> > >
> > > * rshift as the total number of blank bits in the u64
> > > * lshift as number of blank bits left of the bitfield in the u64
> > >
> > > Take a moment to consider why that is true by consulting the above
> > > diagram.
> > >
> > > With this insight, we can how define the following relationship:
> > >
> > > bitfield
> > > _
> > > | |
> > > 0.....00XXX0...00
> > > | | | |
> > > |______| | |
> > > lshift | |
> > > |____|
> > > (rshift - lshift)
> > >
> > > That is, we know the number of higher order blank bits is just lshift.
> > > And the number of lower order blank bits is (rshift - lshift).
> > >
> >
> > Nice diagrams and description, thanks!
>
> Thanks!
>
> >
> > > Finally, we can examine the core of the write side algorithm:
> > >
> > > mask = (~0ULL << rshift) >> lshift; // 1
> > > nval = new_val; // 2
> > > nval = (nval << rpad) & mask; // 3
> > > val = (val & ~mask) | nval; // 4
> > >
> > > (1): Compute a mask where the set bits are the bitfield bits. The first
> > > left shift zeros out exactly the number of blank bits, leaving a
> > > bitfield sized set of 1s. The subsequent right shift inserts the
> > > correct amount of higher order blank bits.
> > > (2): Place the new value into a word sized container, nval.
> > > (3): Place nval at the correct bit position and mask out blank bits.
> > > (4): Mix the bitfield in with original surrounding blank bits.
> > >
> > > [0]: https://reviews.llvm.org/D133361
> > > Co-authored-by: Eduard Zingerman <eddyz87@gmail.com>
> > > Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
> > > Co-authored-by: Jonathan Lemon <jlemon@aviatrix.com>
> > > Signed-off-by: Jonathan Lemon <jlemon@aviatrix.com>
> > > Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
> > > ---
> > > tools/lib/bpf/bpf_core_read.h | 34 ++++++++++++++++++++++++++++++++++
> > > 1 file changed, 34 insertions(+)
> > >
> > > diff --git a/tools/lib/bpf/bpf_core_read.h b/tools/lib/bpf/bpf_core_read.h
> > > index 1ac57bb7ac55..a7ffb80e3539 100644
> > > --- a/tools/lib/bpf/bpf_core_read.h
> > > +++ b/tools/lib/bpf/bpf_core_read.h
> > > @@ -111,6 +111,40 @@ enum bpf_enum_value_kind {
> > > val; \
> > > })
> > >
> > > +/*
> > > + * Write to a bitfield, identified by s->field.
> > > + * This is the inverse of BPF_CORE_WRITE_BITFIELD().
> > > + */
> > > +#define BPF_CORE_WRITE_BITFIELD(s, field, new_val) ({ \
> > > + void *p = (void *)s + __CORE_RELO(s, field, BYTE_OFFSET); \
> > > + unsigned int byte_size = __CORE_RELO(s, field, BYTE_SIZE); \
> > > + unsigned int lshift = __CORE_RELO(s, field, LSHIFT_U64); \
> > > + unsigned int rshift = __CORE_RELO(s, field, RSHIFT_U64); \
> > > + unsigned int rpad = rshift - lshift; \
> > > + unsigned long long nval, mask, val; \
> > > + \
> > > + asm volatile("" : "+r"(p)); \
> > > + \
> > > + switch (byte_size) { \
> > > + case 1: val = *(unsigned char *)p; break; \
> > > + case 2: val = *(unsigned short *)p; break; \
> > > + case 4: val = *(unsigned int *)p; break; \
> > > + case 8: val = *(unsigned long long *)p; break; \
> > > + } \
> > > + \
> > > + mask = (~0ULL << rshift) >> lshift; \
> > > + nval = new_val; \
> > > + nval = (nval << rpad) & mask; \
> > > + val = (val & ~mask) | nval; \
> >
> > I'd simplify it to not need nval at all
> >
> > val = (val & ~mask) | ((new_val << rpad) & mask);
> >
> > I actually find it easier to follow and make sure we are not doing
> > anything unexpected. First part before |, we take old value and clear
> > bits we are about to set, second part after |, we take bitfield value,
> > shift it in position, and just in case mask it out if it's too big to
> > fit. Combine, done.
> >
> > Other than that, it looks good.
>
> I mostly left it there for the cast. Cuz injecting the `unsigned long
> long` cast made the line really long. How about this instead?
>
> diff --git a/tools/lib/bpf/bpf_core_read.h b/tools/lib/bpf/bpf_core_read.h
> index a7ffb80e3539..7325a12692a3 100644
> --- a/tools/lib/bpf/bpf_core_read.h
> +++ b/tools/lib/bpf/bpf_core_read.h
> @@ -120,8 +120,8 @@ enum bpf_enum_value_kind {
> unsigned int byte_size = __CORE_RELO(s, field, BYTE_SIZE); \
> unsigned int lshift = __CORE_RELO(s, field, LSHIFT_U64); \
> unsigned int rshift = __CORE_RELO(s, field, RSHIFT_U64); \
> + unsigned long long mask, val, nval = new_val; \
> unsigned int rpad = rshift - lshift; \
> - unsigned long long nval, mask, val; \
> \
> asm volatile("" : "+r"(p)); \
> \
> @@ -133,9 +133,7 @@ enum bpf_enum_value_kind {
> } \
> \
> mask = (~0ULL << rshift) >> lshift; \
> - nval = new_val; \
> - nval = (nval << rpad) & mask; \
> - val = (val & ~mask) | nval; \
> + val = (val & ~mask) | ((nval << rpad) & mask); \
sgtm
> \
> switch (byte_size) { \
> case 1: *(unsigned char *)p = val; break; \
>
>
> Thanks,
> Daniel
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