diff options
| -rw-r--r-- | Documentation/networking/filter.txt | 15 |
1 files changed, 9 insertions, 6 deletions
diff --git a/Documentation/networking/filter.txt b/Documentation/networking/filter.txt index 5032e1263bc9..e6b4ebb2b243 100644 --- a/Documentation/networking/filter.txt +++ b/Documentation/networking/filter.txt @@ -1142,6 +1142,7 @@ into a register from memory, the register's top 56 bits are known zero, while the low 8 are unknown - which is represented as the tnum (0x0; 0xff). If we then OR this with 0x40, we get (0x40; 0xbf), then if we add 1 we get (0x0; 0x1ff), because of potential carries. + Besides arithmetic, the register state can also be updated by conditional branches. For instance, if a SCALAR_VALUE is compared > 8, in the 'true' branch it will have a umin_value (unsigned minimum value) of 9, whereas in the 'false' @@ -1150,14 +1151,16 @@ BPF_JSGE) would instead update the signed minimum/maximum values. Information from the signed and unsigned bounds can be combined; for instance if a value is first tested < 8 and then tested s> 4, the verifier will conclude that the value is also > 4 and s< 8, since the bounds prevent crossing the sign boundary. + PTR_TO_PACKETs with a variable offset part have an 'id', which is common to all pointers sharing that same variable offset. This is important for packet range -checks: after adding some variable to a packet pointer, if you then copy it to -another register and (say) add a constant 4, both registers will share the same -'id' but one will have a fixed offset of +4. Then if it is bounds-checked and -found to be less than a PTR_TO_PACKET_END, the other register is now known to -have a safe range of at least 4 bytes. See 'Direct packet access', below, for -more on PTR_TO_PACKET ranges. +checks: after adding a variable to a packet pointer register A, if you then copy +it to another register B and then add a constant 4 to A, both registers will +share the same 'id' but the A will have a fixed offset of +4. Then if A is +bounds-checked and found to be less than a PTR_TO_PACKET_END, the register B is +now known to have a safe range of at least 4 bytes. See 'Direct packet access', +below, for more on PTR_TO_PACKET ranges. + The 'id' field is also used on PTR_TO_MAP_VALUE_OR_NULL, common to all copies of the pointer returned from a map lookup. This means that when one copy is checked and found to be non-NULL, all copies can become PTR_TO_MAP_VALUEs. |