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patch-author-guide: explain "inline" and implicit "notrace" 

The kernel's definition of "inline" implies "notrace" and that may lead
to unexpected kpatch-build errors.  Explain why that is and how to work
around the issue.

Fixes #983.

Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>
This commit is contained in:
Joe Lawrence 2019-12-05 14:48:29 -05:00
parent ea348164f6
commit 1e2f73216f
1 changed files with 60 additions and 2 deletions
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62
doc/patch-author-guide.md
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@ -567,8 +567,8 @@ In some patching cases it might be necessary to completely remove the original
function to avoid the compiler complaining about a defined, but unused
function. This will depend on symbol scope and kernel build options.
Other issues
------------
"Once" macros
-------------
When adding a call to `printk_once()`, `pr_warn_once()`, or any other "once"
variation of `printk()`, you'll get the following eror:
@ -597,3 +597,61 @@ For example, a `pr_warn_once()` can be replaced with:
pr_warn("...");
}
```
inline implies notrace
----------------------
The linux kernel defines its own version of "inline" in
include/linux/compiler_types.h which includes "notrace" as well:
```
#if !defined(CONFIG_OPTIMIZE_INLINING)
#define inline inline __attribute__((__always_inline__)) __gnu_inline \
__inline_maybe_unused notrace
#else
#define inline inline __gnu_inline \
__inline_maybe_unused notrace
#endif
```
With the implicit "notrace", use of "inline" in patch sources may lead
to kpatch-build errors like the following:
1. `__tcp_mtu_to_mss()` is marked as inline:
```
net/ipv4/tcp_output.c:
/* Calculate MSS not accounting any TCP options. */
static inline int __tcp_mtu_to_mss(struct sock *sk, int pmtu)
{
```
2. the compiler decides not to inline it and keeps it in its own
function-section. Then kpatch-build notices that it doesn't have an
fentry/mcount call:
```
% kpatch-build ...
tcp_output.o: function __tcp_mtu_to_mss has no fentry/mcount call, unable to patch
```
3. a peek at the generated code:
```
Disassembly of section .text.__tcp_mtu_to_mss:
0000000000000000 <__tcp_mtu_to_mss>:
0: 48 8b 87 60 05 00 00 mov 0x560(%rdi),%rax
7: 0f b7 50 30 movzwl 0x30(%rax),%edx
b: 0f b7 40 32 movzwl 0x32(%rax),%eax
f: 29 d6 sub %edx,%esi
11: 83 ee 14 sub $0x14,%esi
...
```
This could be a little confusing since one might have expected to see
changes to all of `__tcp_mtu_to_mss()` callers (ie, it was inlined as
requested). In this case, a simple workaround is to specify
`__tcp_mtu_to_mss()` as `__always_inline` to force the compiler to do so.