ff672c67ee
On x86-64 the tail call count is passed from one BPF function to another
through %rax. Additionally, on function entry, the tail call count value
is stored on stack right after the BPF program stack, due to register
shortage.
The stored count is later loaded from stack either when performing a tail
call - to check if we have not reached the tail call limit - or before
calling another BPF function call in order to pass it via %rax.
In the latter case, we miscalculate the offset at which the tail call count
was stored on function entry. The JIT does not take into account that the
allocated BPF program stack is always a multiple of 8 on x86, while the
actual stack depth does not have to be.
This leads to a load from an offset that belongs to the BPF stack, as shown
in the example below:
SEC("tc")
int entry(struct __sk_buff *skb)
{
/* Have data on stack which size is not a multiple of 8 */
volatile char arr[1] = {};
return subprog_tail(skb);
}
int entry(struct __sk_buff * skb):
0: (b4) w2 = 0
1: (73) *(u8 *)(r10 -1) = r2
2: (85) call pc+1#bpf_prog_ce2f79bb5f3e06dd_F
3: (95) exit
int entry(struct __sk_buff * skb):
0xffffffffa0201788: nop DWORD PTR [rax+rax*1+0x0]
0xffffffffa020178d: xor eax,eax
0xffffffffa020178f: push rbp
0xffffffffa0201790: mov rbp,rsp
0xffffffffa0201793: sub rsp,0x8
0xffffffffa020179a: push rax
0xffffffffa020179b: xor esi,esi
0xffffffffa020179d: mov BYTE PTR [rbp-0x1],sil
0xffffffffa02017a1: mov rax,QWORD PTR [rbp-0x9] !!! tail call count
0xffffffffa02017a8: call 0xffffffffa02017d8 !!! is at rbp-0x10
0xffffffffa02017ad: leave
0xffffffffa02017ae: ret
Fix it by rounding up the BPF stack depth to a multiple of 8, when
calculating the tail call count offset on stack.
Fixes:
|
||
|---|---|---|
| Documentation | ||
| LICENSES | ||
| arch | ||
| block | ||
| certs | ||
| crypto | ||
| drivers | ||
| fs | ||
| include | ||
| init | ||
| ipc | ||
| kernel | ||
| lib | ||
| mm | ||
| net | ||
| samples | ||
| scripts | ||
| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .clang-format | ||
| .cocciconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
| .mailmap | ||
| COPYING | ||
| CREDITS | ||
| Kbuild | ||
| Kconfig | ||
| MAINTAINERS | ||
| Makefile | ||
| README | ||
README
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.