On 22.07.2025 17:02, Roger Pau Monné wrote: > On Wed, Jul 16, 2025 at 06:31:03PM +0200, Jan Beulich wrote: >> On 16.07.2025 18:00, Roger Pau Monne wrote: >>> In a livepatch payload relocations will refer to included functions. If >>> that function happens to be a replacement for an existing Xen function, the >>> relocations on the livepatch payload will use the newly introduced symbol, >>> rather than the old one. This is usually fine, but if the result of the >>> relocation is stored for later use (for example in the domain struct), >>> usages of this address will lead to a page-fault once the livepatch is >>> reverted. >>> >>> Implement a second pass over relocations once the list of replaced >>> functions has been loaded, and fixup any references to replaced functions >>> to use the old symbol address instead of the new one. There are some >>> sections that must be special cased to continue using the new symbol >>> address, as those instances must reference the newly added livepatch >>> content (for example the alternative patch sites). >> >> This is what I was fearing, when you first mentioned the problem (and the >> plan) to me. What I don't see is why you do your fixing up regardless of >> relocation type. Relative relocations within the payload ought to be fine >> to not override? At which point some of the special casing may already no >> longer be necessary. >> >> (Later) Except that if code uses PC-relative addressing to determine a >> pointer to store into some struct, that'll appear as a relative relocation >> type, too. But then you may have a bigger problem: When referencing and >> referenced code are in the same section and in the same translation unit, >> the assembler could avoid emitting a relocation altogether. You would see >> nothing to fix up ... > > The only way for the referencing and referenced code to be in the same > function would be for the function to reference itself, which should > be quite rare? I don't recall seeing any code in Xen where a function > stores a pointer to itself. > > Otherwise each function is in a separate section, and hence references > to functions should always use a relocation.
Oh, right, I forgot about -ffunction-sections being in use. Jan
