On Wednesday, December 14, 2016 12:18:12 AM John Baldwin wrote: > Author: jhb > Date: Wed Dec 14 00:18:12 2016 > New Revision: 310045 > URL: https://svnweb.freebsd.org/changeset/base/310045 > > Log: > Use casts to force an unsigned comparison in db_search_symbol(). > > On all of our platforms, db_expr_t is a signed integer while > db_addr_t is an unsigned integer value. db_search_symbol used variables > of type db_expr_t to hold the current offset of the requested address from > the "best" symbol found so far. This value was initialized to '~0'. > When a new symbol is found from a symbol table, the associated diff for the > new symbol is compared against the existing value as 'if (newdiff < diff)' > to determine if the new symbol had a smaller diff and was thus a closer > match. > > On 64-bit MIPS, the '~0' was treated as a negative value (-1). A lookup > that found a perfect match of an address against a symbol returned a diff > of 0. However, in signed comparisons, 0 is not less than -1. As a result, > DDB on 64-bit MIPS never resolved any addresses to symbols. Workaround > this by using casts to force an unsigned comparison.
I am somewhat unsure of why this worked on other architectures. amd64 treated ~0 as 0xffffffff which when assigned to a 64-bit register was zero-extended. i386 also used 0xffffffff, but it used an unsigned comparison (jae instead of jge). The kernel linker API returns an unsigned long for the diff, so I do think using db_addr_t for this type is probably the right solution in the long term. > Probably the diff returned from db_search_symbol() and X_db_search_symbol() > should be changed to a db_addr_t instead of a db_expr_t as it is an > unsigned value (and is an offset of an address, so should fit in the same > size as an address). Also, in case it wasn't clear, this fixes resolution of addresses to names in MIPS64 stack traces in DDB as well as when using 'x', etc. -- John Baldwin _______________________________________________ svn-src-all@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/svn-src-all To unsubscribe, send any mail to "svn-src-all-unsubscr...@freebsd.org"
